266 COPE. [Vou. III. 
their office, or the molars are in contact with each other or with 
the food. They then relax, and their temporal muscle continues 
the upward press- 
ure, but draws the 
ramus backwards 
to the limit set by 
the adjacent parts, 
causing the act of 
mastication. 
AA mith eect ‘ot 
the development 
of the incisors and 
of the proal mas- 
tication, is seen in 
the position of the 
molar teeth. The 
indefinitely repeat- 
ed strain and press- 
ure applied to the 
superior molars 
from forwards and 
below has evident- 
ly caused a gradual 
extension of the 
maxillary bone 
backwards, so that 
pterygoid fossa; c, internal pterygoid plates; d, fossa in the last molars oc- 
basioccipital bone; ¢, external auditory meatus; f, mastoid CUupy a position 
process; g, occipital condyles; 4, tympania bulla; after much posterior to 
Hall*and Wyman. fiat baintek they 
do in other orders of Mammals. This is especially the case in 
such forms as Bathyergus, Arvicola, and Castoroides (Fig. 86), 
where the last molars are below the temporal fossa, and poste- 
rior 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 forwards for the lower, and downwards and _ back- 
wards for the upper. The mechanics of this change of direction 
Figure 87. — Castoroides ohioensis Foster; two-fifths 
nat. size; skull from below. Fig. a, incisine foramen; 4, 
1 Proceedings Academy Philadelphia, p. 66, Figs. 8, 6 and /- 
