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 

 Anouiodontia, have distinct 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 ^^'hales 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 is 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 Tillo- 

 doutia and in Diprotodont ilarsupials, 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 



