264 COPE. [Vo. III. 
would tend to keep the mouth permanently open, were it not 
for the possibility of slipping the lower jaw backwards as it 
closed on the upper. This backward pressure had undoubtedly 
existed, and has operated from the earliest beginning of the 
growth of the rodent incisors. The process has been precisely 
the opposite of that which has occurred to the Carnivora, where 
the pressure has been ever forwards owing to the development 
of the canines.t The progressive lengthening of the incisors 
through use has been dwelt on by Professor Ryder (4c.). The 
Figure 86.— Castoroides ohioensis Foster, skull, right side; two-fifths nat. size. 
Fig. a, inferior insertion of masseter muscle; @, fossa inside of ascending ramus; 
c, external auditory meatus; d, incisors; e, foramen infraorbitale; from Hall and 
Wyman. 
posterior pressure on the lower jaw produced by its closing on 
the upper, has been increased directly as the increase in the 
length of the incisors, especially those of the lower jaw. 
The first effect of this posterior pressure will have been to 
slide the condyle of the mandible posteriorly over the post- 
glenoid process, if any were present, as is probable, in the 
Bunotherian ancestor of the rodent. Continued repetition of 
the movement would probably push the process backwards so 
as to render it ineffective as a line of resistance, and ultimately 
