8 Origin of the Dentition of the Rodentia. 
cle is to draw the ramus backwards and upwards, a movement 
which is commenced so soon as the inferior incisor strikes the apex 
of the superior incisor on the posterior side. By this muscle the 
inferior molars are drawn posteriorly and in close opposition to the 
superior molars. Connected with this movement, probably as an 
effect, we find the coronoid process of the mandible to have become 
gradually reduced in size, to complete disappearance in some of the 
genera, e.g., of Leporide. In these genera the groove-like insertion 
of the temporal muscle develops as the coronoid process disappears. 
Fig. 5. Ischyromys typus Leidy, natural size, from the White 
river beds of Colorada; onpas, from the Report G, z Geol. 
Surv, Terrs. a,b,c, cranium; d, mandible from above 
As third and fourth effects of the posterior position of the lower 
jaw, we have the development of the internal pterygoid and mas- 
seter muscles and their insertions and origins. The angle of the 
ramus being forced backwards, these muscles are gradually stretched 
backwards at their insertions, and their contraction becomes more 
antero-posterior in direction than before. The internal pterygoid 
mpap specially developed, and its point of origin, the pterygoid 
ossa, becomes much enlarged. The border of the angle of the 
ath becomes more or less inflected. In their effect on the move- 
