No.2.] THE HARD PARTS OF THE MAMMALIA. 269 
inferior molars include a wider expanse than the superior, though 
this applies in some instances more to the direction of the roots 
rather than the position of the crowns. In Lepus the lower 
jaw is the narrower. I have termed the two types of anisog- 
nathism hypanisognathism (Lepus, Diplarthra) and epanisog- 
nathism (Caviide). The following genera display these char- 
aetehst 
Hypanisognathous. Lsognathous. Epantsognathous. 
Lepus. Arvicola. Hystricide. 
Capromys. Castor. 
Caviide. 
In conclusion I will say that it is satisfactorily proven to my 
mind that nearly all of the peculiarities of the rodent dental 
system and manner of mastication, are the mechanical conse- 
quences of an increase in the length of the incisor teeth. And 
the increase of the length of these teeth has been due to their 
continued use, as believed by Ryder. 
b. Monotremata Multituberculata. The structure of the 
dentition of this suborder is in many respects like that of 
the Rodentia in the known forms. The incisors in the Plagi- 
aulacidze, Chirogidze, and Polymastodontidz have structure and 
functions generally sim- 
ilar to those of the Ro- 
dentia. The result in 
the form and function of 
the molar dentition has 
been similar to that ob- 
served in the Rodentia. 
The postglenoid process BS 
is probably absent in —< 333 : 
these animals; in any 
case the mandibular Figure g1.— Chirox plicatus Cope, palate and 
condyle is rounded, and molar teeth from below; three-halves nat. size; 
Ter at Gian erse Prof from Puerco bed of New Mexico; from American 
i ; : 
Naturalist, 1887, p. 566. 
Pye .Oshborn,) ‘has 
pointed out to me that mastication was performed by a fore- 
and-aft movement of the inferior molars on the superior in 
Plagiaulacidz. This was no doubt the case in the other families 
named. The resulting structure of the crown is, however, dif- 
ferent, and needs explanation. The molar teeth present conical 
