40 MAMMALIAN DENTITION 



the growth of the tooth itself its occurrence in the Alligator 

 precedes the eruption of the tooth. 



The Reptiles form a large and miscellaneous assemblage of 

 creatures but their most numerous and more primitive repre- 

 sentatives are extinct, We must not expect to find among the 

 living examples of this class the types Avhich will interest us 

 most. In fact the Reptiles may be subdivided into primitive, 

 mammal-like and bird-like forms, of which all the first and 

 second groups and many of the third are extinct. Modern 

 Reptiles are all more or less closely allied to the Birds and it is 

 interesting to note that the teeth in many of them, the Turtles 

 for example, have been replaced by horny coverings for the 

 jaws as in Birds. We now know that several at least of the 

 ancestral forms of Birds possessed truly reptilian teeth. 



A rapid survey has shown us the following suggestive facts. 

 Teeth already occur in the lower vertebrate forms. They are 

 at first simple conical outgrowths of surface epithelium which 

 may or may not be attached in one of various ways to the un- 

 derlying jaw. In higher forms the projecting part or crown 

 may become subdivided into more than one point and this sub- 

 division is liable to occur with greater frequency in the teeth 

 situated on the jaw near the areas of muscular attachment 

 rather than in those located near the middle line in the front of 

 the mouth. Certain teeth, as shown in the Alligator, become 

 larger and more powerful than the rest and these characters 

 are also shared by the replacing teeth. Lastly the imbedded 

 part or root of the tooth becomes implanted secondarily in a 

 socket in the jaAv bone. With these observations in mind we 

 are iioav ready to commence the study of the origin of the 

 mammalian dentition itself. 



