234 COPE, (Vou. ITI. 
against the superior over any backwards pressure, has left the 
posterior internal cusp of the triangle of the inferior molar with- 
out contact or consequent functional use. It has, consequently, 
gradually disappeared, having become small in the highest Cani- 
de, and wanting in some Mustelidz, and all Felidz. The 
heel of the same tooth has had a similar history. With the 
diminution in size of the first superior tubercular, with which it 
Figure 60.— Aelurodon savus Leidy; diagram representing coadaptation of 
crowns of superior and inferior molars in mastication; lines and lettering as in Fig. 58. 
comes in opposition in mastication, its functional stimulus also 
diminished ; and it disappeared sometimes a little sooner (Feli- 
dz) and sometimes.a little later (Hyaenidze) than that tooth. 
The specialization of one tooth to the exclusion of others as 
a sectorial, appears to be due to the following causes. It is to 
be observed in the first place that when a carnivore devours a 
carcass, it cuts off masses with its sectorials, using them as 
shears. In so doing it brings the part to be divided to the angle 
or canthus of the soft walls of the mouth, which is at the front 
of the masseter muscle. At this point the greatest amount of 
force is gained, since the weight is thus brought, immediately 
to the power, which would not be the case were the sectorial 
situated much in front of the masseter. On the other hand, the 
sectorial could not be situated farther back, since it would then 
be inaccessible to a carcass or mass too large to be taken into 
the mouth. 
The position of the sectorial tooth being thus shown to be 
dependent on that of the masseter muscle, it remains to ascer- 
tain a probable cause for the relation of the latter to the dental 
series in modern Carnivora. Why, for instance, were not the 
last molars modified into sectorial teeth in these animals, as in 
the extinct Hyzenodon, and various Creodonta. The answer ob- 
viously is to be found in the development of the prehensile char- 
acter of the canine teeth. It is probable that the gape of the 
