Palaeontology of the human dentition ten structural stages in the evolution of the cheek teeth.код для вставкиСкачать
American Journal of Physical Anthropology VOLUME IX OCTOBER-DECEMBER NUMBER 4 PALAEONTOLOGY OF THE HUMAN DENTITION TEN STRUCTURAL STAGES I N THE EVOLUTION OF THE CHEEK TEETH1 WILLIAM K. GREGORY CONTENTS ........ PAGE 401 ............... 404 ARISE ON INNER ................... 404 IKE; LOWERS FIT INTO FOURTH STAGE,CRETACEOUS: PRETRITUBERCULAR UPPER MOLARSWITH CONNATE PARA-ANDMETACONES .................................... 411 FIFTHSTAGE,LOWEREOCENE: TYPICALTRITUBERCULAR UPPER MOLARS AND TUBERCULO-SECTORIAL LOWER MOLARS. ......................... 415 APPEARANCEOF THE HYPOCONE AND RESIXTHSTAGE,MIDDLEEOCENE: DUCTION OF THE PARACONID; FIRSTWIDENING OF THE TALONID. ..... (MI SEVENTH STAGE,UPPEREOCEKE: UPPERMOLARS LAR; LOWER MOLARS WITH P R O MI N E N T HYPOCONULI EIGHTHSTAGE,MIOCENE: WIDENING OF THE LOWER OF THE Dryopithecus PATTERN. .. ...................... NINTHSTAGE,PLEISTOCENE: FURT IDENING OF THE LOWER TENTHSTAGE, RECENT:REDUCTION OF THE HYPOCONE OF M', DELAYED ERUPTION OF M9, Loss OF THE Dryopithecus PATTERN, M1 AND M, DOMINANT ......................................................... 423 SUMMARY .......................................................... 425 REFERENCES TO LITERATURE. ........ . . . . . . . . . . 426 INTRODUCTION The human upper and lower jaws in normal occulsion are part of a mechanism, the wonder and beauty of which are nowhere more fully appreciated than by the members of this Congress. Were these complex relations of upper and lower cusps and surfaces created perfect, a t one stroke, have they always been as we now find them, or have they attained their present status after a long and gradual series of modifications from more simple beginnings? And how far, in the present imperfect state of our knowledge, can we begin to discern an orderly succession of stages in the evolution of the dental apparatus? 'Read before the First International Orthodontic Congress, New York City, August 16-20, 1926. AM. J. PRYS. ANTHROP., 1926, Val. IX, NO.4. 401 402 WILLIAM K. GREGORY Since the evolution of the orthodontic relations of the human upper and lower teeth is thus clearly a phase of the general study of the evolution of man, it may be fortunate for me that I can say what I have t o say now, before a new constitutional amendment m&es the whole topic legally taboo and our next Congress in this country subject to closure by agents of the Federal Government. At least in New York at the present time it is still reasonably safe to lecture in public on the evolution of man, but all over this broad land fiery zealots, with the energy of the Hebrew prophets, are calling upon their followers to throw down the idols of the evolutionists. Thundering into loud speakers from a thousand pulpits, they brand the scientific doctrine of the evolution of man as a flimsy hypothesis. Professing their desire t o purge and purify true science, they denounce as “pseudo-scientists” all university professors and laboratory workers who dare to teach anything contrary to the traditional belief that Adam was literally made out of the dust of the ground and Eve from one of Adam’s ribs. But it is vain for them t o kick against the pricks. They may make it illegal to teach evolution in state-supported universities, colleges and schools, but they cannot stop the endless discoveries of facts that have made the gradual evolution of the body and mind of man one of the most reasonable and fully documented inferences in the whole field of the biological sciences. The radio orators naturally feel no embarrassing doubts as to their own competence to dispose of the subject of the evolution of man after they have read a few popular works on the subject and have got hold of a few citations of so-called authorities who have expressed opinions supposed to be damaging t o this or that point of the evolutionist’s argument. The scientist, however, is necessarily more modest. He realizes that thousands of important details of the story of human evolution must ever remain in doubt, and that at the present time science has reached no brief clear-cut picture as to the general mechanism of evolution, that is, the precise manner in which Natural Selection, the Environment, and other factors finally overcome the conservative forces of heredity and induce a definite shift from one structural level t o the next. The scientist, moreover, recognizes that there is no royal road to a firsthand knowledge of the broader aspects of the origin and evolution of man. He perceives that the evolution of man is only a special case of the evolution of animals in general, and so he must first labor unceasingly to disentangle and classify the myriad twigs and branches of the great tree of vertebrate life. Fortunately a small army of investigators, PALAEONTOLOGY OF THE HUMAN DENTITION 403 namely, students of systematic zoology, comparative anatomy and palaeontology have long been fruitfully engaged in this field. Thousands of studies in the anatomy and classification of recent and extinct vertebrates, all quite unknown to the pulpit and public press, have supplied the outlines of a tolerably detailed picture of the main branches and twigs of vertebrate life and of the principal stages in the ascent from fish to man. From a wide synthesis of such studies it has been learned that the evolution of the vertebrates may be sketched under the following heads : first, the successive changes in the locomotor apparatus; secondly, the evolution of the respiratory, vascular and alimentary systems, the last including the masticatory apparatus; third, the evolution of the reproductive organs and methods; fourth, the evolution of the coordinating organs of the peripheral and central nervous systems. Scores of millions of years ago in Devonian times we meet the relatives and forerunners of the fishes with limbs and lungs, the hardy pioneers that first struggled up out of the water on to the land. Next, in the time of the Coal Measures, we find both the varied amphibians not yet wholly freed from the water and the earliest common ancestors of the reptiles and of all higher types. Coming on to the Permian and Triassic ages, each many millions of years in duration, we encounter several grades of organization among the mammal-like reptiles and some of these almost deserve to be callFd pro-mammals, so progressive are they in locomotor and masticatory apparatus. Proceeding to the Jurassic ages we come upon true mammals, with the beginnings of the tritubercular type of molar teeth ; and quite recently Cretaceous Placental Mammals have been discovered in Mongolia, which as we shall see, supply a long-needed link in the evolution of the molars. Passing to the Lower Eocene we find early representatives of that great order the Primates, which was destined eventually to give rise to man, and which even at that far-distant epoch was already distinguished by its superior development of eye and hand and brain. In the Lower Oligocene of Egypt we meet the first ancestors of the branch which was later to subdivide into the anthropoid apes on the one hand and the races of man on the other, and in the Miocene we encounter traces of various true anthropoids, among which are some that, according tomy interpretation of the evidence, stand in or quite near to the common stem of man and the modern anthropoids. In the Lower Pliocene our anatomical records, apart from one or two debatable finds, are a blank, but in the Pleistocene we have in gradually ascending levels, first the Pithecanihrops, a true man although in a low stage of development, 404 WILLIAM K. GREGORY secondly, the Dawn Man or Eoanihropus of England, thirdly, the Heidelberg man, fourthly, the Neanderthal man, and finally the Cro-Magnon, true Homo sapiens, and his successors of the late Palaeolithic, Neolithic, Copper, Bronze and Iron Ages. At each stage of this long line of ascent we must analyse the known osteological and dental characters and endeavor to discover what forms lie in or near the line of human descent and which ones are leading off in various directions to the specialized side lines. The ten structural stages in the evolution of the human cheek teeth described above are all represented by known forms, extending in time through ascending geological horizons from the Permo-Carboniferousto the Recent. Not one of them is in any sense hypothetical. The drawings depicting the verifiable facts of observation have been made with the most painstaking care by a skilled artist, Mrs. Helen Ziska, under the author’s close and constant supervision. FIRST STAGE, PERMO-CARBONIFEROUS: UPPER AND LOWER CHEEK TEETH ALIKE, CANINIFORM With such a general sketch before us we may take up the special subject of the present paper, namely, the pakeontology, or development in past ages, of the human dentition. And first let us pass over all the earlier stages in the evolution of the teeth, when in the ancestors of the air-breathing fishes the tough skin around the mouth, bristling with small shagreen denticles, through a change of function became useful in holding on to small living prey. We must begin rather with that prlnitive reptilian stage represented by the small pelycosaurian or therornorph reptiles of the Permo-Carboniferous (Fig. 1, Ia), in which the upper teeth were simple and laniariform with compressed enamelcovered crown, each tooth with a single root embedded in a socket. In such a reptile the teeth of the premaxillary and maxillary bone of the upper jaw overhang and alternate with the similar teeth of the lower jaw. This was the beginning of relations that still hold to some extent in the human jaws. It need hardly be said that, in such a simple mechanism, when the lower jaws are drawn upward by the action of the temporal muscle-mass the food is both pierced and sheared by the pointed, sharpedged teeth. SECOND STAGE, TRIASSIC : “PROTOCONE” AND METACONID ARISE ON INNER SIDE OF UPPER AND LOWER MOLARS The next stage of evolution is illustrated in the Pro-mammal or Cynodont reptiles of the Triassic of South Africa. For in them (Fig. 1, PALEONTOLOGY OF THE HUMAN DENTITION 405 Ic) the dentition has become differentiated into incisors, canines, premolars and molars, and the available evidence indicates that the succession of the teeth was reduced to two sets, corresponding to the deciduous and permanent series of mammals. In many genera of the Cynodonts both the upper and the lower cheek teeth were all compressed and shearing, with one or more accessory cusps on the margins of the shearing blade, but in some other genera of Cynodonts constituting the family Diademodontidz (Fig. 1, II), while the original tips of the upper still overhung the tips of the lower cheek teeth, the upper molar teeth in the middle of the tooth row were extended transversely on the palatal side, while the corresponding lower molar crowns had acquired a subcircular outline with a transverse ridge across the summit of the crown. I n such a dentition the upper and the lower teeth are markedly unlike, the uppers being transversely oval with a tuberculate transverse ridge, the lowers much smaller and circular but also with a transverse ridge. These Diademodont Cynodonts, if not themselves the direct ancestors of the mammals, were a t least nearly related to such ancestors; they already exhibited the beginning of the power of developing a cuspidate molar crown, which became so marked a characteristic of the mammals themselves, and they were also extraordinarily mammal-like in many significant morphological characters of the skull. It is therefore significant that in all the Cynodonts the original tip of the molar crowns lies on the outer or buccal side in both the upper and the lower teeth; and that the upper and lower cheek teeth fall in inter-locking series such that each lower articulates with two uppers; also that the basal swellings on the lingual side of both upper and lower molars constitute the main overlapping parts of the crowns. THIRD STAGE, JURASSIC : UPPER AND LOWER MOLARS UNLIKE, LOWERS FIT INTO INTERDENTAL EMBRASURES The exact stages between the pro-mammal with compressed shearing teeth and the pretritubercular molars of Cretaceous mammals are still in doubt, notwithstanding the famous theory of Cope and Osbom as t o the origin of the upper and lower triangles by rotation or migration of the accessory cusps outward in the upper and inward in the lower teeth. Unfortunately our present evidence does not permit us to confirm this theory, but on the contrary it indicates that the tritubercular molars of later mammals did not originate in this manner. A fundamental postulate of the Cope-Osborn theory was that the original tip of the ancestral reptilian tooth gave rise in the mammals, in the upper molars t o themain WILLIAM K. GREGORY 406 A U Fig. 1 PALAEONTOLOGY OF THE HUMAN DENTITION b CI 5' 407 408 WILLIAM K. GREGORY FIG.1. Ten Structural Stages in the Evolution of the Human Dentition, from Ascending Geological Horizons. I. Substage a. Permo-Carboniferous. Mycteroseurus, primitive Theromorph Reptile. After Williston. Substage b. Permian. Scylacosaurus, primitive Mammal-like Reptile. After Broom. Substage c. Triassic. Cynognathus, advanced Mammal-like Reptile. After Seeley. 11. Triassic. Diademodon, advanced Mammal-like Reptile. Mainly after Seeley. Occlusion diagram by author. 111. Jurasic. Pantotherian (primitive pro-Placental). Kindness of Dr. G. G. Sim son. Occlusion diagram by Simpson. I&. Cretaceous. Pre-Trituberculate, Deltatheridium. From the original specimen. Occlusion diagram by author. V.. Lower Eocene. Primitive Placental, Didelphodus. From the original specimen. Occlusion diagram by author. FIG.2. Ten Structural Stages in the Evolution of the Human Dentition (con’t)’ VI. Middle Eocene. Primitive Primate, Pronycticebus. m e r Grandidier. Ocdusion‘diagram by author. VII. Upper Eocene. Advanced Tarsioid Primate, Microclwrus. After Stehlin. Occlusion diagram by author. VIII. Miocene. Primitive Anthropoid Primate, Dryopithecus. Upper molars mainly after Pilgrim; lower molars from type of Dryopithecus cautleyi. Occlusion dia am by author and Milo Hellinan. 1%. Pleistocene. Primitve Man, Mousterian. From stereoscopic photographs by Professor J. H. McGregor and from the published photographs by Weinert and by Virchow (m3). Occlusion diagram by author. X. Recent. Modern Man, White. From the original specimen. Occlusion diagram by author. internal cusp, which was therefore called the protocone, and in the lower molars to the main external cusp, called the protoconid. All the available evidence, both from the ancestral Cynodonts and from the mammals of the Cretaceous and Tertiary ages, now indicates that at least in the lines leading to the later mammals the supposed reversal of the relationship of the upper 2nd lower original tips did not take place, but that on the contrary the original tips remained on the buccal side in both the upper and the lower teeth, while the inner basal part of the crown became extended toward the inner side t o give rise to the crushing portions of the crown. Another postulate of the Cope-Osborn theory was that in the supposed stage of “reversed triangles” the upper and lower molar crowns were exactly alike in form; but it now seems clear that at a very early stage the upper and lower molars became quite unlike, the uppers being larger, roughly triangular in form, with large interdental embrasures into which the small sharply triangular lowers fitted. At this stage (Fig. 1, 111) , represented by the Jurassic Pantotheria originally described by Professor Marsh and now most ably restudied by Dr, G. G. Simpson, the original tip of the ancestral reptilian tooth in the upper molar was PALAEONTOLOGY OF THE HUMAN DENTITION 409 apparently represented by the main high cusp in the middle of the premolars and molar crowns, a cusp which was destined t o give rise later to the mesiobuccal and distobuccal cusps (para-and metacones). From the anteroexternal (mesiobuccal) comer projected a rounded buttress, the parastyle, surmounted by a raised rim or cingulum, while from the posteroexternal (distobuccal) comer projected the metastyle. The internal angle of the triangular base of the crown was formed by a rounded protuberance terminating in a low cusp, the mesiopalatal cusp, or socalled protocone. The front side of the triangular basal section is relatively short and more transverse in position, the posterointernal (distolingual) side is longer and more oblique. Both these sides oppose the principal moving blades of the lower molars. The lower molar crowns of this stage consist of two radically distinct parts, an elevated triad of cusps, the trigonid, and a small low posterior extension, the incipient heel or talonid. An important difference from the upper molars is that the homologue of the original tip is not central but external in position. The cusps of the trigonid of the lower molars in the Osbomian system of nomenclature are called protoconid (main external), paraconid (anterointernal), metaconid (posterointernal), and for convenience these names are retained, but with the express reservation that according to present evidence the cusps so designated are not severally homologous with the similarly named cusps of the upper molars. If we reject, as seems necessary, the Cope-Osborn theory of the origin of the upper and lower triangles by the folding in opposite directions of simple triconodont teeth, and if we accept the evidence cited by various authors and tending to show that in both upper and lower teeth the original cusps remained on the outer or buccal side, then in the lower molars the first appearance of the paraconid and metaconid as distinct cusps must have been associated with the transverse, or obliquely transverse, widening of the crowns; and both these cusps arose in sibu, the one at the anterointernal extension of the base of the crown and the other as a swelling on the inner or lingual slope of the original tip or protoconid. Meanwhile the so-called protocone or internal cusp of the upper molar is simply a cusp or swelling on the inner or palatal extension of the crown. It was not the original tip but it probably arose as far back as in the Diademodont forerunners of the mammals. In the occulusion diagrams (Fig. 1, 11, 111,IV) we can see the intimate functional relations of these parts of the upper and lower molars. The protocones of the upper molars became differentiated, we may 410 WILLIAM K. GREGORY infer, at the same time with the metaconids of the lower molars. Even in later mammals the metaconid blade shears in front of the anteroexternal blade of the protocone V, while the tip of the metaconid of the lower molar grinds against the anterior side of the protocone of the upper. In primitive insectivorous and carnivorous mammals the protocones are displaced forward, while the blade on the posterior slope of the protocone shears past the blade on the antercexternal face of the metaconid. This situation is already developed in the Jurassic Pantotheria (Fig. 1, 111). The protocone of the upper molars very early came into functional relations with another element of the lower molars, namely the talonid or heel. In the predecessors of the mammals the upper teeth were set in a crowded series and there were probably few or no interdental embrasures. Moreover the pressure of the lower jaw seems to have been chiefly vertical and directly transverse, not obliquely transverse. The Triconodont mammals of the Mesozoic ages remained apparently in this stage of evolution. But the Pantotherian mammals of the Jurassic (Fig. 1, 111) had already advanced to the stage where pressure of the lower jaw in an oblique anteroexternal direction accompanied a forward displacement of the internal extension or protocone of the upper molars, with a concomitant upgrowth of the paraconid and development of the paraconid blade, and finally with the opening up of the interdental embrasures. Meanwhile the protoconid of the lower molars, from being directly above the median forking between the anterior and posterior roots, had shifted forward along with the protocone of the uppers, so that it finally came to lie more above the anterior root, leaving room onitsposterior slope for the backward growth of a new and highly important structure, the talonid. This from the first impinged on the anterior slope of the protocone of the next succeeding upper molar, but by further growth the first cusp of the talonid, namely the entoconid, worked itself around behind the tip of the protocone. By this time each upper and lower molar consisted of two functionally analogous parts: first, a shearing V and secondly, an overlapping heel. The shearing V of the upper tooth was formed by the anterior and posterior slopes of the protocone, more or less connected respectively with similar blades on the anterior and posterior comers of the crown (paraand metastyle). At this stage the large original tip of the upper molars (the future paracone) held aloof from articulating with any part of the lower molars, simply fitting into the interdental embrasures on the outer PALAEONTOLOGY OF THE HUMAN DENTITION 411 sides of the lower molars and pressing the food against the nearby blades on its own base and on the sides of the lower teeth. The shearing V of the lower molar had its tip, the protoconid, on the outer side and its sides (the paraconid and metaconid) directed inward. Here then we have the true functional reversal of upper andlower triangles which apparently misled Osborn and Cope into homologizing the upper and the lower reversed V’s; but as already intimated, the reversed upper and lower v’s both have the primitive apex of the tooth on the outer side of the crown, and in the upper the primitive tip stands not at the internal angle but in the middle of the V, while in the reversed lower V the original tip of the crown is located at the fork of the V. The overlapping or crushing portions of the upper and lower teeth in this stage consisted in the lower molar of the small talonid, with its sole cusp the entoconid, and in the upper molar of the protocone. the tip of which already fitted into the basin of the talonid as it does in nearly all later mammals. Nevertheless I was formerly in error in supposing that the protocone of the upper and its functional mate the talonid of the lower were of equal age. The protocone apparently arose as early as the Diademodont premammalian stage and accompanied a transverse widening of the crowns; the talonid arose later in an early Pantotherian stage and its appearance was made possible by an anteriorward displacement of the metaconid, associated with an anterior displacement of the protocone of the upper molar. Thus by Lower Jurassic times the upper and lower cheek teeth of mammals, as illustrated in the Pantotheria, had already attained rather complex interrelationships, and such a dentition as that of Amphitheriam, which was not dissimilar to that shown in Fig. 1, 111,had in it the morphological potentiality of the highly diverse arrangements and patterns of the molars that are found in existing placental andmarsupial mammals. FOURTH STAGE, CRETACEOUS : PRETRITUBERCULAR U P P E R MOLARS WITH CONNATE PARA- AND METACONES Until quite recently there was a considerable structural gap and an enonnous chronological gap in the record of dental evolution between the Jurassic Pantotheria and the swarming placental mammals of the Eocene and later ages. Owing to this gap in our records it has been in doubt, for example, whether the internal extension of the protocones of the Pantotheria was truly homologous with the so-called protocones of the molars of later mammals, whether the protocones of later mammals arose puri passu with the development of the talonids, and so forth. 413 WILLIAM K. GREGORY But these doubts have at last been resolved, in my own mind at least, through the fortunate discovery by the Third Asiatic Expedition of the American Museum of Natural History, under the leadership of Roy Chapman Andrews, of the long-sought Cretaceous mammal skulls. These priceless specimens, which have recently been described in a joint paper by myself and Dr. G. G. Simpson of Yale University, still retain certain primitive features inherited from the Jurassic Pantotheria: thus their main high central cusps are still flanked on the outer sides by prominent parastyle and metastyle projections, while the basal section of the crown is obliquely and asymetrically triangular with the internal apex displaced anteriorly. On the whole the Cretaceous mammals are already nearer in dental structure to the later placental mammals than to the Jurassic Pantotheria. In one of them, which we have named Deliatheridium petrituberculare, as indeed also in the others, the upper molars (Fig. 1, IV, Fig. 2) show a further progression beyond the primitive Pantotherian condition in the direction of those of typical later mammals. In the first place, the total number of postcanine teeth has been much reduced, from about eleven or twelve to six or seven, the reduction apparently having occurred through the loss of the posterior molars, so that seven larger teeth now occupy the whole tooth row instead of twelve small ones. Secondly, the interdental embrasures of the upper teeth have widened t o accommodate the enlarged talonid of the lower teeth. Thirdly, the talonids of the lower molars have now captured and definitely engage with the entire protocone of the upper molars, the latter being reduced to the condition of a pestle that works in the basin of the talonid. Fourthly, the main central cusp of the upper molar has assumed a more dominant position, since the blades on its anteroexternal and posteroexternal slopes now shear past the blades on the correspondFIG.3. Palate of Primitive ing slopes of the protoconids. Fifthly, Placental Mammal (Deltathrthe main central cusp, the original “repidium frretritubercwhre) from tilian cone” is now in process of being t h e Cretaceous of Mongolia. x 2/1 divided into two cusps, still connate at the base, the para- and metacones (mesiobuccal, distobuccal), Another of these Mongolian Cretaceous genera described by us (Zalambdalestes, Fig. 4 ) , although more in the line of ascent to the PALAEONTOLOGY OF THE HUMAN DENTITION 413 Zalambdodont insectivores than to the higher Insectivora, shows a further advance in the direction of later mammals in that the trigonid is now completely displaced to the forward part of the crown, while the talonid is widened transversely and is now even wider than the trigonid. In correlation with this transverse widening of the talonid the main FIG.^. Right Lower Cheek Teeth of Primitive Insectivore (Zalambdalestes Eeclzei) from the Cretaceous of Mongolia. X 3/1. W FIG.5. Occlusal diagram of ZaZamb dalestes lechei X 5/1. external cusp of the upper molar, already subdivided into the para- and metacones, is retreating toward the outer border of the crown (Fig. 5). These Mongolian Cretaceous genera of placental mammals (Fig. 6 , A, B, C) combine the characters of the insectivores and carnivores of the Eocene and later ages t o such a degree that we regard them as standing in or quite close to the common ancestry of these two orders. And it has long since been inferred from numerous comparative studies that this A B C FIG.6. Skulls of Cretaceous mammals from Mongolia. A. Deltetheridium pretrituberculare. X 1. B. Zalambdalestes lechei, young. X 1. C. Zabmbdalestes lechei, extremely old X 1. hitherto undiscovered primitive insectivore-carnivore group also constituted the common stock of all the later placental orders, including the Primates . It would be too much to expect that the first discovered Cretaceous WILLIAM K. GREGORY 414 placental mammals should be the very ones that would tend to bridge the gap between the Jurassic Pantotheria and the Eocene and later placental mammals and yet such is the fortunate fact. It had already been inferred by several authors from comparative studies of Eocene and later mammals that the main high cusp of the upper molars, standing in line with and behind the similar main cusp of the premolars and likewise supported equally by the two main roots on the outer side of the crown, was serially homologous with that cusp; in other words, that the paracone and not the so-called protocone represented the summit of the original reptilian crown. But in the Cope-Osborn theory of reversed triangles this fact was ignored, although it was tacitly admitted that in the lower molars the main high cusp of the molars, the protoconid, was homologous with the main high cusp of the premolars in front of it. This inherent inconsistency of the Cope-Osborn theory was exposed by me in 1916, and in 1922 I showed that the occlusal relations of the upper and lower cheek teeth made it far more probable that in the upper molars the homologue of the original reptilian cone was the paracone or A B FIG.7. Relations of Main Tips of Molars and Premolars in a Primitive Cretaceous Placental (Deltatheridium pretrituberculare). A. Oblique rear view of left lower cheek teeth, showing t h e protoconids of the molars in line with the tips of the premolars. B. Oblique rear view of right upper cheek teeth, showing the high paracone in line with the tips of t h e premolars and the “protocone” as a low process from t h e base of t h e paracone. the combined para- plus metacone. The newly discovered Mongolian Cretaceous placental mammals afford strong support to this view, namely that in the upper teeth the original reptilian tip lies on the paracone and that in the lower molars it is the protoconid. These relations may be seen at a glance in Fig. 7 ; hereafter the burden of prooE PALAEONTOLOGY OF THE HUMAN DENTITION 415 must be upon anyone who would maintain that in the upper jaw the original reptilian tip shifts as we pass backward from the main high cusp to the internal basal extension, the so-called protocone. F I F T H STAGE, LOWER EOCENE : TYPICAL TRITUBERCULAR U P P E R MOLARS AND TUBERCULO-SECTORIAL LOWER MOLARS From the Cretaceous Mongolian placentals to the highly vaned placentals found in the Basal and Lower Eocene of North America is but a short step. Already in the Lower Eocene and perhaps even earlier the Order of Primates had become separated from other mammals. The existing Tree Shrews (Tupaiidae) of Borneo and adjacent regions seem to be living and but little modified survivors of the basal Primate stock, so that after many studies on their anatomy and osteology certain authors classify them as the first division of the Primates rather than as a highly progressive family of Insectivores. While they have become more or less specialized in their front teeth, their cheek teeth have retained many features seen in the Cretaceous placentals and in other primitive placental mammals of Eocene times. But to illustrate the passage from the Cretaceous t o the Eocene, in the line of ascent toward man, I have chosen a form that is not a primate at all but is of so generalized a type that according to Dr. W. D. Matthew it can most conveniently be referred to the Order Insectivora. I refer to the Lower Eocene genus Didelphodus (Fig. 1, V). I n this genus we see very clearly several distinct advances upon the conditions characteristic of the Mcngolian Cretaceous mammals. This dentition affords an ideally central type for the derivation of all specialized molar patterns of later placental mammals, although it must be borne in mind that since Didelphodus is a Lower Eocene rather than a Basal Eocene or even Upper Cretaceous form, it was even in its own day a relic of still oldeiforms and that it lived beside more progressive contemporaries that had already attained more advanced modifications of the dentition in several directions. Didelphodus then, representing a primitive placental stage of the dentition, had advanced beyond the more primitive Deltatheridiurn of the Cretaceous in the fallowing respects: first, it was no longer in a pre-tritubercular stage, since the para- and inetacones of its upper molars were now well separated, thus producing the typical tritubercular upper molar, which Cope and Osborn rightly chose as the starting point for the more complex niclar patterns of all higher placental mammals. Secondly, the lower molars had attained the typical tuberculo-sectorial WILLIAM K. GREGORY 416 condition with a nearly balanced development of the trigonid and talonid. That this combination of tritubercular upper molars and tuberculosectorial lower molars was in very truth the starting point for the diverse molar pattenis of later mammals would need no emphasis if addressed to the few living pal2eontologists who have devoted their lives to the study of the swarming families of Eocene and later fossil and recent mammals. But unhappily the published evidence for this statement is scattered in scores of technical papers and the main bulk of the evidence can hardly ever be published at all by reason of its very extensiveness. It is therefore still possible even for certain eminent students of anthropology and human morphology to treat with equal scepticism all theories of the early evolution of the molar teeth of mammals and to imagine that they can safely start de nmo and invent a new theory each one for himself. But to return to Didelphodus, it does, as I have said, present ideally primitive upper and lower molar patterns. Every one, but one, of the standard main cusps of the typical primitive placental molar is present and normally developed. The proto-, para- and meta- cones, the protoconule and metaconule and the para- and meta- styles are all present and functioning normally, as may be seen in the occlusion diagram (Fig. 1, v>. The one molar cusp which normal placental mammals now have and which Didelphodus had not yet developed is the hypocone, a later derivative of the posterointernal region of the crown. In Didelphodus the well developed trigonids still fit into the interdental embrasures, the talonid had not yet outstripped the trigonid in size, and its hypoconid had not yet pushed its way laterally beyond the line of the inner bases of the protoconids. SIXTH STAGE, MIDDLE EOCENE: APPE-4RANCE O F THE HYPOCONE AND REDUCTION O F THE PARACONID; FIRST WIDENING OF T H E TALONID. Nature’s beautiful mechanisms sooner or later get thrown out of balance and seek new adjustments through the overgrowth and crowding of cedain parts and the retreat and final disappearance of other parts. This phenomenon is clearly illustrated in Fig. 2 , VI, representing one of the numerous Primates of the Middle Eocene and quite representative in a general way of all its relatives. For in this form the primitive trigon of the upper molar has become modified by the addition of a fourth main cusp, the hypocone, which in this case is derived from a PALAEONTOLOGY OF THE HUMAN DENTITION 417 swelling on the posterior cingulum or basal ledge. The upgrowth of this hypocone tends to reduce and finally to obliterate the interdental embrasure and to transform a triangular tritubercular upper molar into a quadrangular quadritubercular one. This process has not yet affected the third molar, is just beginning in the first, and is well advanced in the second molar. Meanwhile the talonid of the lower molars has increased rapidly in size so that it now surpasses the trigonid; its hypoconid pushing laterally has passed the line of the protoconids and invaded the space formerly occupied by the main high central cusp; while the para- and metacones, derivatives as we have seen of the primitive central cusp, have not only parted widely from each other but have retreated laterally before the advance of the hypoconid. Meanwhile the paraconid is apparently being crowded out of existence, a t any rate, it retreats before the backwardly-growing talonid of the preceding tooth and before the inwardlygrowing hypocone of the upper molars. Thus, as first perceived by Cope and Osborn, we have the upper molars transformed from the tri- to the quadritubercular stage and the lowers from the tuberculo-sectorial into the tubercular stage with four main cusps, the protoconid and hypoconid on the outer side, the meta- and entoconids on the inner side. A fifth small cusp, the hypoconulid, on the median posterior border, had already appeared in the primitive placentals (Fig. 1, V). In the earlier Primates it was poorly developed on the first and second lower molars, but large and prominently develcped on m3; i t occluded on or slightly behind the posterior basal cingulum of the upper molar especially in m3, where it formed a powerful piercing organ since it was located far back near the fulcrum and having a small area, its piercing power was correspondingly high. In this Middle Eocene p r k a t e (Pronycticebus, Fig. 2, VI) we observe that while the first and secc id upper premolars are small, the third and fourth are tending to be su )equal and bicuspidate. A further emphasis of this difference together m th a crowding of the whole tooth row might result in the entire elbinatit n of the first and second premolars, so that the originally third premolar 1 .odd be in contact with the canine. This condition is realized in certain progressive tarsioid primates of the Upper Eocene (e.g., Microchczrus, Fig. 2, VII) and in all the Old World monkeys, anthropoid apes and man. WILLIAM K. GREGORY 418 - SEVENTH STAGE, UPPER EOCENE: UPPER MOLARS (ml,m2)QUADRI- TUBERCULAR; LOWER MOLARS WITH PROMINENT HYPOCONULID Microcharw itself is distinctly too specialized in certain particulars to lie in the direct line of human ascent. Nevertheless it belongs to a division, the Tarsioidea, which in many respects is progressive. Its skull is distinctly progressive toward the higher primate type and as investigations multiply it seems more and more probable that the' line leading to the higher primates including man, branched off from the tarsioid stem before the latter acquired its various aberrant specializations. Therefore it seems legitimate to consider the molar patterns of Microcharus (Fig. 2, VII) in the present connection, since in many ways they are intermediate between the primitive primate stage typified by Pronycticebus and the common stem leading to the anthropoids and man. First, then, Microcharus shows a marked advance beyond Pronyciicebus in the development of the hypocones, which have now very nearly obliterated the interdental embrasures. Concomitantly the paraconids of the second and first molars are nearly or quite gone, but the paraconid of the first molar is still well developed in harmony with the feeble development of the posterointernal cusp of the fourth upper premolar. Secondly, the talonids now considerably surpass the trigonids in size, the hypoconids being predominant over the protoconids. Accordingly the para- and metacones of the upper molars are larger and even further separated than they were in Pronycticebus and lie still nearer to the outer border of the crown. This outer border from its great prominence in the earlier stages is thus greatly reduced, the external cingulum being the last trace of its former extensive development. A specialization of Microcharus is the great development of the metaconule, which occludes behind the enlarged hypoconid and occupiesthe field left vacant by the retreat of the metacone. Another specialization is the doubling of the hypoconulids in the lower molars, which occlude in the space in front of and between the metaconule and the hypocone. EIGHTH STAGE, MIOCENE: WIDENING OF THE LOWER MOLARS; APPEAR- Dryopithecus PATTERN. The upper teeth (Fig. 2, VIII) of the primitive anthropoid Dryopithecus from the Miocene of India and Europe exhibits a marked advance beyond earlier Primates and in the human direction in many particulars. In the first place the two premolars remaining out of the original four, namely p3, p4,are both fully bicuspid and have become closely similar in form. In the upper molars all the cusps have lost their crested shearing ANCE OF THE PALAEONTOLOGY OF THE HUMAN DENTITION 419 form and have become obtusely conical. They have lost all trace of the original outer border of the crown and they have lost the protocondes, retaining however in a’ somewhat modified form the anterior cingular ridge connecting with the anterior crest of the protocone. The metacones have become somewhat smaller than the paracones and do not project so far laterally. The metaconule, no longer distinct, has apparently been fused with the base of the metacone, the well developed hypocone has reduced but not wholly obliterated the interdental embrasure. The third molar has acquired an irregular outline, usually broader on the inner than on the outer border. Meanwhile the lower cheek teeth have undergone profound modifications. The anterior lower premolar (correspondingto the third of the primitive dentition) has remained compressed and is only incipiently or potentially bicuspid. The posterior premolar, on the contrary, is definitely bicuspid with a tendency to become molariform by the development of the talonid. The most conspicuous advance in the molars lies I or onule a FIG.8. Names of the cusps of the upper and lower molars of Dryopithecus, according t o the Osbornian System. in the marked changes in the proportions; the width of the tooth in proportion to its length has materially increased in comparison with the earlier Primates, especially in m2. The ridges on the cusps, that formerly formed the shearing blades, are vestigial or absent, the cusps themselves having become very large‘and conical and being arranged in two main pairs, the first pair (protoconid-metaconid) transverse, the second pair (hypoconid-entoconid) obliquely transverse, with the entoconid a little behind the hypoconid. The hypoconulid varies somewhat in the different species of Dryopithecus but especially on m2 is a prominent conical cusp lying obliquely behind the hypoconid. With the complete loss of the paraconid, the whole trigonid basin has been reduced to a narrow fissure, the “fovea anterior;” while a forward displacement of the protoconid, together with the widening of the whole tooth and the flattening down of all the cusps, has effected a great in- 430 WILLIAM I(. GREGORY crease in size of the talonid basin, which now forms the great central fossa of the tooth. With the reduction in height of the trigonid and the levelling of the general surface of the crown there has been an almost complete loss of the old tuberculo-sectorial pattern and functions. The shearing-grinding effect of bluntly conical cusps grinding past each other has taken the place of the shearing effects of delicate sharp-edged blades of the earlier trigonid, while the crushing-shearing-grinding function of the talonid has become dominant. With the loss of the sharp blades on the trigonid and talonid the conical cusps are no longer integrated into the primitive tuberculo-sectorial pattern, but a new pattern called the Dryopithecus pattern has grown up in place of it. This pattern (Fig. 2, VIII) consists of the collocation of the five main cusps in the positions already noted and with their bases defined by certain deep sulci, which during development appear along zones of contact between the expanding centers of growth of the five main cusps. An essential feature of the Dryopithecus pattern is that there is such a zone of contact between the posteroexternal slope of the metaconid and the anterointernal slope of the hypoconid. The occlusion diagram of Dryopithecus brings out even more closely the functional significance of the characters already noted. AS compared with the occlusion diagrams of earlier forms it reveals the marked increase in width of the lower as compared to the upper teeth, the complete loss of the prbitive shearing blades of the trigonid, the enhanced functional importance of the talonid and of the hypoconulid, the lateral growth of the hypoconid between the paracone and metacone. As a whole there is decidedly less alternation between the upper and lower teeth than there was in the primitive stages; that is, the reduction of the trigonid and the increase of the talonid causes each lower tooth to articulate more with the correspondingly numbered upper tooth than with the preceding upper tooth. But beneath these changes the old fundamental relations of the main cusps of the upper and lower molars are still very apparent. Thus the remnant of the trigonid still lies between two upper teeth, thehypoconid articulates between the para- and the metacone (although further laterad), the protocone still falls between the metaconid and the entoconid and the latter articulates on the anterior slope of the hypocone. NINTH STAGE, PLEISTOCENE : FURTHER WIDENING O F THE LOWER MOLARS The oldest human dentition known in which complete upper and lower sets belong t o one individual is that of the fossilized skull of a young man of the Neanderthal race (Homo neanderthalensis) discovered PALAEONTOLOGY OF THE HUMAN DENTITION 421 a t Le Moustier in France and described originally by Klaatsch as Homo rnousferiensis hauseri (Fig. 2, IX). From stereoscopic photographs taken by my colleague Professor J. H. McGregor and from an excellent cast of the specimen presented to the American Museum by Dr. J. Leon Williams, I am enabled to give diagnostic drawings of the upper and lower cheek teeth and to construct a diagram of the upper and lower teeth in occlusion. The European anthropologists who have described this dentition stressed its resemblances to and difierences from other specimens referred to the Neanderthal race and to Homo sapiens and have said nothing about the whole series of fundamental characters which it has inherited from earlier primates and which the present series of drawings brings into sharp relief. As to the upper cheek teeth, most of the description of the upper teeth of the anthropoid Dryopifhecus, given above, would apply word for word to this human specimen. Thus on each side of the palate there are not only the same number of cheek teeth, but also the same total number of principal cusps on each corresponding tooth, as follows: PJ P’ Ma Mz Ma Total 2 2 4 4 4 4 4 16 Pa P* MI Ma Ma Total 2 3 4 3 4 5 5 5 5 5 5 20-21 20-21 Dryopithecus. . . . . . . . . . . . . . . . . . . . 2 LeMoustier.. . . . . . . . . . . . . . . . . . . 2 4 16 Moreover, each one of the corresponding cusps and ridges has the same general character in the Mousterian youth as it has in Dryopithecus. The lower cheek teeth of Le Moustier likewise show the same number of teeth and the same total number of principal cusps as in Dryopithecus, as follows: Dryopithecus.. . . . . . . . . . . . . . . . . . . LeMoustier.. . . . . . . . . . . . . . . . . . . 2 Except in the third lower molar, in which the fundamental pattern has been obscured by secondary grooves and wrinkles, as it sometimes is in the Chimpanzee and still more in the Orang, the Dryopithecus pattern is still evident in the lower molars of Le Moustier, which retains the essential contact between the adjacent bases of the metaconid and hypoconid. Moreover the occlusion diagrams reveal the extraordinary degree of correspondence between Dryopilhecus and Le Moustier in all fundamental characters, so that it may be stated as a fact that in the general pattern and in the occlusal relations of its cheek teeth, the primitive anthropoid Dryopithecus is far nearer to the primitive human stage represented by Le Moustier than it is to the early primate Pronycticebus. For example, the tendency toward an end-to-end rather than a broadly 42% WILLIAM K. GREGORY alternating relation of the upper and lower molars, already established in Dryopithecus, is retained in Le Moustier, which is in this respect much nearer to Dryopithecus than the latter is t o Pronycticebus. Finally, many of the differences between Dryopdhecus and Le Moustier represent further advances in the same direction in which the former differs from primitive primates. In Le Moustier, for example, pa has progressed considerably toward the molar pattern since its posterior moiety and its incipient hypoconid and entoconid are now better developed than they were in Dryopithecus; p3 from being at most incipiently bicuspid has become completely so, its main axis now being transverse instead of anteroposterior. But that this difference between Le Moustier and Dryopithecus may fairly be classed as merely a progressive character of the former and not a divergence tending to remove Dryopithecus from the line of human ascent, is indicated by the wide degree of variability in the form of p3 among recent chimpanzees and gorillas, in some of which ps retains largely the compressed Dryopithecus form, while in others the crown of p3 has been rotated so that the direction of its main axis is obliquely labiolingual, its lingual border bearing a strong cusp, so that the tooth as a whole approaches the bicuspid type. In the lower molars of Le Moustier the broadening of the crown has progressed much further than it had in Dryopithecus; m3 has completely lost its tuberculo-sectorial character and has gone a step further on the road toward degeneration, delayed eruption and eventual disappearance. The hypoconulid, but moderately developed in Dryopifhecus, is larger in Le Moustier, especially in ml. Turning again to the occlusion diagram we note in Le Moustier the further tendency of the hypoconulid to crowd into the territory formerly occupied by the hypoconid, with an associated tendency in ml for the hypoconid to expand laterally. But in addition to all these characters in which Le Moustier has either retained the pure Dryopithecus status or progressed further in the same direction in which Dryopiithecus had advanced beyond the earlier primates, we witness many new features in Le Moustier which were in general associated with a profound change in the form of the dental arch as a whole. For whereas in primitive anthropoids the jaw is relatively narrow transversely, the opposite halves of the lower jaw and the opposite rows of cheek teeth being more or less parallel, in man the mandible has been greatly widened and shortened, the front part of the alveolar border crowded backward, so that the lower incisors and canines are no longer procumbent, but in the more advanced races even PALAEONTOLOGY OF THE HUMAN DENTITION 428 pass the vertical. Meanwhile the canine tooth has diminished in length and its tip has finally withdrawn beneath the closed up row of upper teeth, while the forepart of the dental arch has shrunk transversely so that the premolar series incline more toward the midline, all these changes producing the characteristic paraboloid upper and lower arches which had already been attained in the Pleistocene species of man represented by Le Moustier. It is true that apart from the doubtful exception of the Talgai palate we do not yet possess the structurally intermediate stages between the Dryopithecus condition with narrow dental arch and enlarged canines and the condition seen in Le Moustier and other early human races in which the canine is definitely human. But in view of the extraordinary resemblances in the cheek teeth already noted, taken in connection with the cumulative evidence from comparative anatomy as t o the derivation of the human family from a primitive anthropoid stock, there can be little reasonable doubt that there once were such intermediate stages, in which the lower canine was smaller and more erect than in the primitive anthropoid and the front part of the jaw already in course of being crowded backward. Indeed the mandible of the Piltdown race may, when its front part is better known, afford just such an intermediate stage. The lower molars of the Mousterian youth also exhibit another major advance toward the modernized human stage, since its second molar already shows signs of substituting the cruciform for the Dryopithecus pattern. That is, with the crowding forward of the entoconid the transverse sulcus in front of the entoconid has been brought nearly in line with the transverse sulcus between the protoconid and the hypoconid. Meanwhile the pairing of the hypo- and entoconid in a transverse row prepares the way for the completion of the cruciform pattern in the present stage of evolution. TENTH STAGE, RECENT: ERUPTION OF M;, REDUCTION O F T H E HYPOCONE OF M2, DELAYED LOSS OF THE Dryopiflzecus PATTERN, M' AND M x DOMINANT The cheek teeth of modern man, as the members of this Congress well know, vary widely both in size and form, but the dentition of the female Bedouin illustrated in Fig. 2, X. may be taken perhaps as fairly representative for the white race. That this dentition is in some respects considerably simplified and reduced, as compared with the more vigorously developed dentitions of infra-human primates, there can be no reasonable doubt. Professor $24 WILLIAM K. GREGORY Cope long ago noted that in certain races, especially the Esquimaux but also in allied Mongolian races, there was a tendency for the second upper molar to exhibit only the three “primary cusps,” the fourth or hypocone being reduced or absent. , Cope spoke of this phenomenon as a “reversion” to a “lemurine” “tritubercular” stage of evolution but the use of the words reversion and lemurine only brings in an unnecessary and confusing element of mystery. For all stages in the reduction of the hypocone may be observed in any large collection of human skulls. Moreover, the occlusal relations of the three main cusps are substantially the same as in the quadritubercular second molars of Le Moustier and Dryopithecus, to both of which the resemblance is far closer than to the tritubercular second upper molar of primitive Eocene primates. The third upper molar in this fully adult individual is well developed but was not yet in contact with the third lower molar, this delay in assuming its functional responsibilities presaging the final degeneration of these teeth attained in some human jaws. Along with the diminution in size of m2,m3, as compared with the corresponding teeth of older types, we observe in this Bedouin the dominance of m*and of its fellow in the lower jaw, which is one of the outstanding features of modernized human dentitions. In the lower molars we now find another conspicuous reduction,namely the loss of the hypoconulid as a distinct cusp,-in which the modernized dentition has sacrificed some of the features slowly won through long geological ages and in which there is a marked tendency toward simplification. The reduction of the hypoconulid has been traced again and again by numerous observers, in various races and individuals. In general this loss is more pronounced in m2than in ml, which although progressive in dmensions is more conservative in pattern than mz. Usually with the loss of the hypoconid of mzthe substitution of the plusshaped or cruciform arrangement of the principal sulci has been completed. As a result of the changes in dimension noted above we find that with regard to anteroposterior length rns is usually shorter than ml it is also longer than ms.This is the direct opposite of the relations obtaining in Dryo@zecus, where the molars typically increase in length from ml to m3, but many intergrading conditions between these extremes have been observed in fossil and recent human jaws. Another marked characteristic of such modernized dentitions as that shown in Fig. 2, X is for the two upper bicuspids to become like each other; a similar tendency is seen in the two lower bicuspids. The PALAEONTOLOGY OF THE HUM.4N DENTITION 4525 result of all these modifications is an apparent simplicity and a degree of resemblance between adjacent teeth which has led some authors t o farreaching theories of the early evolution of the teeth, based upon the development or upon the teratological variations in modernized human teeth. Who would have suspected, from an examination of the human dentition in its present simplified condition, that it had passed through the long and complex series of changes which we have followed above? SUMMARY AKD CONCLUSIONS Unfortunately the detailed labors of palaeontologists and of students of the major classification and evolutiQn of the mammals are so little known to most of their own colleagues in other branches of the biological sciences that few can realize from first-hand knowledge that the study of the evolution of human molar teeth is no longer in the vague stage of fog and uncertainty. The cumulative and ever-widening evidence for Darwin’s view that man is an off-shoot from the base of the anthropoid stem securely links the study of the eyolution of human cheek teeth with the history of dental evolution in the infrahuman primates. And here we have clear documentary evidence, resting on no hypothetical stages, that enables us to follow the patterns of the human dentition backward to the Dryopithecus stage, thence t o the primitive Primates of the Eocene, the dentition of which in turn closely approaches those of many other Eocene phyla that sprang from the central tuberculosectorial Placentals of Basal Eocene times. The discovery of definitely pre-tritubercular Placental mammals in the Cretaceous of Mongolia carries the history backward to a point near or a t the origin of the “stem Placental” mammals, long expected but never hitherto known from fossil specimens. Back of this the long gap to the very beginnings of the future tritubercular stock in the Lower Jurassic still remains to be explored in detail, but even from present evidence little doubt can remain that the relation of reversed triangles between the upper and lower molars was not arrived a t by the steps inferred by Cope and Osborn but took the general course inferred in outline by Wortman and several subsequent authors. The known Jurassic Pantotherian mammals again are advanced far beyond the most advanced mammal-like reptiles of the Triassic, but these in many respects are nearer in structure to the mammals than they are to the stem reptiles of the Permo-Carboniferous. Among the latter Seymouria again divides the difference between the most primitive amphibians and the typical reptiles, while there is the strongest morphological and palaeontological evidence for WILLIAM K. GREGORY 416 connecting the amphibians with some undiscovered group of air-breathing fishes related both to the Fringe-finned Ganoids and the Dipnoans. Reading the story the other way, the ten structural stages in the evolution of the human cheek teeth, as described in the present paper, may be summarized as follows: I. 11. 111. IV. V. VI. VII. VIII. IX. X. Substage a. Permo-Carboniferous. Mycberusaurus, primitive Theromorph Reptile. Substage b. Perrnian. Scyhusaurus, primitive Mammal-like Reptile. Substage c. Triassic. Cynognathus, advanced Mammal-like Reptile. Triassic. D i a d e m d o n , advanced Mammal-like Reptile. Jurassic. Pantotherian (primitive pro-Placental). Cretaceous. Pre-Trituberculate, Deltatheridiunz. Lower Eocene. Primitive Placental, Didelphtodus. Middle Eocene. Primitive Primate, Prunycticebus. Upper Eocene. Advanced Tarsioid Primate, Microchmus. Miocene. Primitive Anthropoid Primate, Dryopithecus. Pleistocene. Primitive Man, Mousterian. Recent. Modern Man, White. REFERENCES TO LITERATURE The older palaeontological literature dealing with the subject under consideration is cited and discussed in “The Ori ‘n and Evolution of the Human Dentition,” by William K. Gregory, Baltimore, 8 2 2 . Consequently only a few of the more important older papers are cited below. 1901-02. Wortman J. L. Studies on the Eocene Mammalia in the Marsh Collection, Peabody Museum. A m . J. Sci., XI-XIV. 11 Pls. 1903. Studies on the Eocene Mammalia in the Marsh Collection, Peabody Museum. A m . J . Sci., XV, 419-436. [The Premolar-Analogy Theory] 1906. Gidley, J. W. Evidence bearing on Tooth Cusp Development. Proc. Wash. Ac. sci., VIII, 91-110, Pls. IV, v. 1907. Osborn, Henry Fairfield. Evolution of Mammalian Molar Teeth to and from the Triangular Type. The Macmillan Company. N. Y. 1910. Gregory, William K. The Orders of Mammals. Bull. Am. Mus. Nut. Hist., XXVII, 1-524. 1916. , Studies on the Evolution of the Primates. Part I. The CopeOsborn “Theory of Trituberculy” and the Ancestral Molar Patterns of the Part 11. Phylogeny of Recent and Extinct Anthropoids with Special Primates. Reference t o the Origin of Man. Bull. A m . Mus. Nut. Hist., XXXV, Art. XIX, 239355. 1925. Simpson, G. G. Mesozoic Mammalia 111: Preliminary Comparison of Jurassic Mammals. Am. J . Sci., X 559-569. 1926. Gre ory, William K. and Milo Hellman. The Dentition of Dryo~thecus and the &gin of Man. Anthrop. Papers Am. Mus. Nat. Hist., XXVIII, Pt. I. .