48 INCISORS OF RODENTS CHAP. 
the Toothed Whales the teeth are homodont, as they are in 
the frog and in most reptiles; on the other hand, some of the 
remarkable reptiles belonging to Professor Huxley’s order of the 
Anomodontia have distinet canines, and show other differentiations 
in their teeth. 
A second characteristic of the mammalian dentition is the 
limited number of the teeth, which rarely exceeds fifty-four. 
Here again the Toothed Whales are an exception, the number of 
their teeth being as great as in many reptiles. In the Mammalia 
the number of the teeth is fixed (excepting of course for ab- 
normalities), while in reptiles there is frequently no precise 
normal. Two regions may be distinguished in every tooth— 
the crown and the root; the latter, as its name denotes, is 
imbedded in the gum, while the crown is the freely-projecting 
summit of the tooth. The varying proportions of these two 
regions of the tooth enables us to divide teeth into two series— 
the brachyodont and the hypselodont ; in the latter the crown is 
developed at the expense of the root, which is small; the 
hypselodont tooth is one that grows from a persistent pulp or, at 
any rate, one that is long open. Brachyodont teeth on the 
contrary have narrow canals running into the dentine. The 
primitive form of the tooth seems undoubtedly to be a conical 
single-rooted tooth, such as 1s now preserved in the Toothed 
Whales and in the canine teeth of nearly all animals. The de- 
velopment of the teeth, that is, the simple bell-shaped form of the 
-enamel organ, seems to go some way towards proving this ; but it is 
quite another question whether we can fairly regard the Whales 
as having retained this early form of tooth. In their case the 
simplification, as is so often the case where organs are simplified, 
seems to be rather degeneration than retention of primitive 
characters. But this is a matter which must be deferred for the 
present. 
The incisor teeth are generally of simple structure and nearly 
always single rooted. In the Rodents, in the extinct Tuillo- 
dontia and in Diprotodont Marsupials, they have grown large, and, 
as has been already stated, they increase in size continuously 
from the growing pulp. These teeth have a layer of enamel 
only on the anterior face, which keeps a sharp chisel-like edge 
upon them by reason of the fact that the harder enamel is worn 
away more slowly than the comparatively soft dentine. The 
