50 CHEEK GEEME CHAP. 
the biting surface ends. The grinding teeth vary from simple 
one-cusped teeth, precisely like canines, to teeth with an 
enormous number of separate tubercles. In the former case it is 
hard to distinguish between incisors, canines, and cheek teeth 
in the lower jaw, where no suture separates the bone. More- 
over it is quite common for the first cheek tooth in the lower 
jaw to have the characters of a canine, while the true canine 
approximates in its form to the antecedent incisors. This is 
so, for instance, with the Lemurs, where the first premolar is 
caniniform, and the canine shares in the curious procumbent 
attitude which distinguishes the lower incisors of many of those 
animals. 
A variable number of the anterior cheek teeth may be little 
more than simple conical teeth; but the rest of the set are 
commonly more complicated. No definite laws can be laid down 
as to the complication of the posterior as compared with the 
anterior set. Broadly speaking, it is purely herbivorous creatures 
in which the least difference can be detected at the two ex- 
tremities, and which are at the same time the most elaborately 
decorated with tubercles and ridges. The converse is true that 
in purely carnivorous animals, including insect- and fish-eating 
forms, there is the greatest difference between the anterior set 
of grinding teeth and those which follow. In these two respects 
such animals as a Lemur and a Rhinoceros occupy the extremes. 
Furthermore, it may be said that omnivorous creatures he, as 
their diet would suggest, in an intermediate position. Generally 
speaking, when there is a marked difference between the first 
premolar and molars at the end of the series, there is a gradual 
approximation in structure of a progressive kind. The tubercles 
become more numerous in successive teeth; but the corollary 
which is apparently deducible from this, 7.e. that the last molar 
is the most elaborate of the series, is by no means always true. 
The last cheek tooth indeed is often degenerate. On the other 
hand, it is very markedly the largest of the series in such diverse 
types as the Elephant, the hog Phacochocrus, and the Rodent 
Hydrochoerus. It is arule that the cheek teeth of the upper jaw 
are more complicated than the corresponding teeth of the lower 
jaw. 
The structure of the cheek teeth is very diverse among the 
Mammalia. Broadly, two types are to be recognised. There are 
