172 Origin of the Specialized Teeth of the Carnivora. [March, 
the lower Eocene, may have been derived from ancestors without 
or with rudimental inner cusps. In any case the effect of use in 
lengthening the cusps appears to have operated in the Carnivora, 
as it has done to a greater degree in the Ungulata; and the lat- 
eral vertical wear would appear to have resulted in the blade-form, 
as transverse wear in the Ungulates has resulted in the plane 
grinding surface. 
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 ob- 
served 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 ofthe 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 masse- 
ter. On the other hand the sectorial could not be situated far- 
ther 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 depen- 
dent on that of the masseter muscle, it remains to ascertain a 
probable cause for the relation of the latter to the dental series 
in modern Carnivora. Why, for instance, were not the last mo- 
lars modified into sectorial teeth in these animals, as in the ex- 
tinct Hyaenodon, and various Creodonta. The answer obviously is 
to be found in the development of the prehensile character of the 
canine teeth. It is probable that the gape of the mouth in the 
Hyaenodons, was very wide, since the masseter was situated rel- 
atively far posteriorly. In such an animal the anterior parts of 
the jaws with the canines had little prehensile power, as their 
form and anterior direction also indicates. They doubtless 
snapped rather than lacerated their enemies. The same habit is 
seen in the existing dogs, whose long jaws do not permit the lac- 
erating power of the canines of the Fe/ide, though more effec- 
tive in this respect than those of the Hyaenodons. The useful- 
ness of a lever of the third kind, depends on the approximation 
of the power to the weight; that is, in the present case, the more 
anterior the position of the masseter muscle, the more effective 
the canine teeth. Hence it appears that the relation of this mus- 
cle to the inferior dental series depended originally on the use of 
