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Tooth replacement in brewer's mole.

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TOOTH REPLSCEAWNT I N BREWER'S MOLE
1%'. KOEERT E A D I E
Dcpartnieiit of Zoology, Cornell Unicersity, Itkaca, S e w P o r k
ONE PLATE (FOUR F ~ G U R E S )
INTRODUCTION
Tooth succession among the insectivores has been described by several workers, notable Leche (1895) and llToodward (1896), and has
been summarized by Weber ( '27). Among the moles, tooth succession
has been studied in Talpa, Scalopus, Condylura and Urotrichus. but
similar information for the genus Parascalops is not on record. The
acquisitioii of suitable stages makes possible the presentation of the
following information on tooth replacement in this genus of moles.
ADULT A S D JUVENILE COSDITIOPL'S
Adult moles of this species ( Parascalops breweri Bachmaii) have
tlie hypothetically complete dental formula of placental inaiirmals :
i."R ., c . 1 pni. $; m. 3;total 44. This map be varied occasionally by
the presence of an extra tooth (Eadie, '39) ~vhileone specinien was
collected which had two extra teeth or a total of forty-six. These additional teeth may he in tlic incisor or premolar series of either jam and
a r e unicuspids iiearly o r quite as large as the regular members of
the series.
I n juvenile moles (8-10 weeks old) that had but recentlj- deserted
their home nest the permanent teeth were riot full.\- erupted aiid some
of the temporary teeth were still in place and visible in the jaws
beside the perniarient teeth. The teniporar:T teeth at this stage a r e
held in place by the gum mcnibraiics onlj- and a w lost in well cleaned
skulls. These teeth a r e veyv small (fig. 1) and it is doubtful that thev
are ever functional. Their penetration of the gums is of inteieest
since in sonie genera of moles (Sculopus and ('ondylura) the milk
teeth never cut the gums, while in Talpa tlie-jT barely break through
(Leche, 1895).
CONDITJONS IS SESTLINC; M O L E S
I n nestling moles approxiniately <5 days old (70 mm. total length)
there a r e no teeth visible on tlie jaw surfaces. Serial sections of these
jaws reveal the development of both the temporary aiid permanent
dentitions.
337
358
W. ROBERT EADIE
The deciduous upper iiicisors appear as well calcified teeth with the
tooth germs of the permanent incisors lying lingual to them. These
deciduous incisors a r e small cone-shaped teeth of which tlie first is the
largest measuring sliglitl!. more than 1 inni. in length while the second
and third arc less than 1 mm. long. The tooth germs of tlie permanent
upper incisors a r e well developed a t this stage although there is yet
no evideiice of the deposition of enamel or dentine (fig. 2 ) . I n correlation with adult proportions the primordium of thc first incisor is much
larger than that of the secoiid or third.
The condition of the deciduous lower incisors aiid of the priinordia
of their permanent successors is mucli as described for the upper jaw
although here there is no appreciable difference in size between the
nienibers of either the deciduous or permanent set at this stage.
Tlie temporary canines of both jaws resemble closely thc second a i d
third temporary incisors in form and size, while the primordia of
the lxmiianent canines are in a stage of development comparable to
that of the permanent incisors.
In both jaws the first premolar of the milk set is a large tooth not
nearly as f a r aclvariced in development as the other members of this
set, although enamel and dentine a r e being deposited. Tliis tooth is
in a stage of developineiit comparable to that of the molars described
below. On the lingual side of each of these first premolars can be seen
a strand of epithelial cells representing the tooth germ for the permanent premolar which has undergone apparent degeneration (fig. 3 ) .
Thus it appears that the first premolar of the adult is a persisting
member of the milk set aiid is not replaced. This condition of the first
premolar parallels that of Talpa europaea (Woodward, 1896) among
the insectirores.
The secoiid and third deciduous premolars of botli jaws and the
fourth in the lower jaw resemble the temporary incisors and canines
i n form aiid size and a r e small, simple, cone-like teeth. Medial to each
lies the well developed prirnordium of a permaimit tooth which is
at the same stage of clerelopmeiit as the priniordia of the permanent
incisors and canines. Tlie deciduous fourth preniolar of the upper jaw
is larger and more complex than tlie preceding milk teeth but is iii a
comparakie stage of development. I t has three roots and a ridge-like
cusp. On its lingual side is located the well formed pririiordiuni of
the permanent tooth.
The molar teeth a r e a t ail advanced stage of development witli wellformed crowns in which eiiainel and dentine are present (fig. 4).
TOOTH REPLACEMEKT I N BREWER’S MOLE
359
DISCUSSION
After examining the available evidence on the milk dentition of the
Insectivora, Woodward (1896) concluded that in this order of manimals there is a distinct tendency toward reduction in the functional
importance of the milk dentition, and that “the living Insectivora are
specialized forms tending towards a Monopliyodoiit condition in which
the preponderating dentition is the replacing or permanent set ”.
The condition in Parascalops where,.with the exception of the persisting first milk premolars, the members of the deciduous series are
minute, spicule-like teeth which arc sooii lost is additional evidence
in support, of this conclusion.
LITERATITURF: CITED
EADIE,
W. R.
1939 B contribution t o the biology of Parascalops breweri. J. Mainm., rol. 20,
no. 2, pp. 1.70-173.
LECHE,W. 1895 Zur Entwickelungsgeschicl~te des Zahnsjstenis der Saugetliiere. Bibliotheca
Zoologica, H e f t 17.
WEBER,M. 1927, 1928 Die Saugetiere. I1 Aufl., Bd. 1, 2. Gustav Fisclier, Jena.
Wooowa~o,hl. F. 1896 Contributions t o the study of the mammalian dentition. Part IT.
Proc. Zool. Sor., London, pp. 5.57-594.
TOOTH REPLACEMENT I N BREFVER’H JI(:IAE
\q. EOBERT EADIE
PLATE 1
1 T i e x of partiall? crnpted pelniiiiiciit teeth of the u p p r jaw in a juvenile mole sliowing
teinporar? incisor still hi place hetwcrii srcc~ndand third peiinaiient incisors (arrow). x 4.
2 Section of the upper jaw of :i liestling mole. Shows well dereloped second teiiiporary
incisor and the enaniel organ of its permanent swcessor lying lingual t o it. X 60.
3 Section of the upper jaw of a nestiing mole. 81ioms first premolar of the milk set i n a
stage of development comparable to that of the molars. Note the degenerate priiiiordiuin
of the first premolar of the permanent set on the lingual side of this tooth (arrow). x 90.
4 First upper molar of liestling mole showing stage of dc.ielopmeiit coinparable to first
milk premolar. x 30.
360
3
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