Origin of the Dentition of the Rodentia. 11 
the inflections are more numerous and profound on opposite sides 
of the molars of the respective jaws. Anisognathism in rodents 
is generally, as shown by Ryder, of the type where the inferior 
molars include a wider expanse than the superior, though this 
Dae aaa tavensis Cope, jaws, two-thirds apie om from Puerco 
a G Kos exico. Figs. a-d, lower jaw; e, upper jaw. Origin 
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. The two types of anisognathism may be termed hyp- 
anisognathism (Lepus, Diplarthra) and epanisognathism (Caviidæ). 
The following genera display these characters :— 
Hypanisognathous. Tsognathous. Epanisognathous, 
Le Arvicola Hystricide. 
Capromys. Castor. 
Caviidee. 
In conclusion I will say that it is satisfactorily proven to my 
mind that nearly all of the peculiarities of the Rodent dental sys- 
. tem, and manner of mastication, are the mechanical consequences 
of an increase in the length of the incisor teeth. And the increase 
in the length of these teeth has been due to their continued use, as 
believed by Ryder. 
