Origin of the Dentition of the Rodentia. 9 
ments of the ramus they oppose that of the temporal muscle, since 
they draw the ramus forwards. They are the effective muscles in 
the use of the incisor teeth, that is, in the opposition of the inferior 
incisors against the superior, from below and posteriorly. Hence 
the great development of the internal pterygoid, and, in a less 
degree, of the masseter. Both muscles tend also to close the jaws, 
but at a different point in the act of mastication from that at which 
the temporal acts. If we suppose the mouth to be open, the action 
of the masseter and internal pterygoid muscles draws the mandible 
forward and upwards until the incisors 
have performed their office, or the molars 
are in contact with each other or with the 
food. They then relax, and the temporal 
muscle continues the upward pressure, nes 
but draws the ramus backwards to the Vj} 
limit set by the adjacent parts, causing 
the act of mastication. fae 
A fifth effect of the development of |} 
the incisors, and of the propalinal masti- § 
cation, is seen in the positions of the molar 
teeth. The indefinitely repeated strain 
and pressure applied to the superior 
molars from forwards and below, has 
evidently caused a gradual extension of 
the maxillary bone backwards, so that 
the last molars occupy a position much 
posterior to that which they do in other sopi Lalay natnrai oise Frome 
orders of mammals. This is especially a, i aen ofsuperior incisor 
the case in such forms as Bathyergus, s wont parts of inferior molars 
Arvicola and Castoroides (Fig. 4), where 
the last molars are below the temporal fossa, and posterior to the 
orbit, 
A sixth effect of the causes mentioned has been referred to by 
Ryder.! This is the oblique direction of the axes of the molar 
teeth. These directions are opposite in the two jaws; upwards 
and forward for the lower, and downward and backwards for the 
Upper. The mechanics of this change of direction from vertical 
in the primitive forms (Sciuride) to oblique in the genera with 
prismatic molars, is simple enough. The inferior crowns, when 
1 Proceed. Acad. Philada., p. 66, Figs. 8, b and f. 
