Origin of the Dentition of the Rodentia. 7 
a line of resistance, and ultimately to flatten it out, and. atrophy 
it. The lower jaw would thus come to occupy that peculiarly 
posterior position which it does in all rodents. 
The anteroposterior (propalinal)? type of mastication becoming 
necessary, an appropriate devel »pment of the muscles moving the 
lower jaw, with their insertions, follows pari passu. As a result 
we see that the insertion of the temporal muscle creeps forwards on 
the ramus, until in the highest rodents (Cavia) it extends along 
the ramus to opposite the first true molars. The office of this mus- 
? Bee Naturalist, Nov., 1987, p. 991, for explanation of the different 
modes of mastication. The propolinal mastication is to be distinguished 
into the proal, from behind forwards (the Proboscidea, Ryder), and the 
palinal from before backwards (the Rodentia, Ryder). 
