Page 444 - My FlipBook

454 DENTAL ANATOMY.
tinguishecl from the Canidce—and, for that matter, from all other
diphyodont monodelphous mammals—by the possession of four true
molars in the lower jaw, thereby giving the formula J. C. \, Pm.
f,
1^, M. f — 46. The only other cases in which there are more than
three true molars normally are found in the marsupials, edentates, and
cetaceans ; and in these two latter orders we have already seen that the
teeth are not generally divisible into incisors, canines, premolars, and
molars, on account of the development of only a single set. In the
marsupials, however, as we shall presently see, the normal number of
Fig. 232.

Skull of Ailnrodon sceviis, Leidy, three-eighths natural size (after Cope).
true molars is four, just as the number three is most common to dip-
hyodont monodelphs. Reduction of the normal number is to be fre-
quently observed in the monodelphs, and, as we have just seen in the
Canidce, occurs in genera otherwise nearly related ; it cannot therefore
be regarded as of more than generic imjjortancc, but there are no cases
known to me in which teeth have been added. On the contrary, I am
firmly of the opinion that not so much as a single tooth has ever been
added to the diphyodont mammalian dentition in the course of develop-
ment, but that specialization has invariably gone in the opposite direc-