6 Origin of the Dentition of the Rodentia. 
Those of the lower jaw closed behind those of the upper in the 
progenitors of the Rodentia (e. g., Esthonyx), as in other Mam- 
malia. Increase of length of these teeth in both jaws would tend 
to keep the mouth permanently open, were it not for the possi- 
bility of slipping the lower jaw backwards as it closed on the 
upper. This backward pressure has 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 in the Carnivora, where the pressure has been 
ever forwards, owing to the development of the canines." The 
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at 
ed 
Mieco aa 
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C 
Fie. 3. Foster, skull, right si ide two-fifths natural size. Fig’ 
a, ar meya jvt masseter masele b, ‘fossa in side of ascending ramus; ic ome 
ernal auditory meatus; d, incisors: e, foramen infraorbitale. From Hall & Wyman, 
progressive lengthening of the incisors through use has been dwelt 
on by Professor Ryder (l. c.). The posterior pressure on the lower 
jaw, produced by its closing on the upper, has been increased 
directly as the increase of the anteroposterior length vi 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 postglenoid pro- 
cess, if any were present, as is probable, in the bunotherian ancestor 
of the rodent. Continued repetition of the movement would prob- 
ably push the process backwards, so as to render it ineffective as 
* 
1 Proceed. Amer. Assoc. Science, 1887 
