42 MAMMALIAN DENTITION 



are broken. Often there is merely a portion of a limb or a 

 mandible exhibiting an imperfect complement of teeth, a frag- 

 mentary skeleton on which to base onr reconstruction of the 

 animal to which it once belonged. 



If then the rocks hold the secret of mammalian ancestry, 

 in what geological stratum may we expect to find it. Some au- 

 thors have conceived the history of Mammals reaching to very 

 early times but it appears probable that we need penetrate no 

 further into the geological past than the Permian epoch to attain 

 our goal. Examining in succession the Mammals of former 

 periods, receding always further into the dim and more imper- 

 fectly known ages of the world's history, or even comparing 

 the more primitive members of the mammalian class existing 

 today with those more advanced, Ave note the occurrence of 

 reptilian features more pronounced and more numerous the 

 more primitive the type. From this fact we realize that Mammals 

 must have been derived originally from reptilian forms al- 

 though from forms very unlike those Reptiles which exist in 

 modern days. 



There must have been a time when certain creatures were 

 transformed from the cold to the warm-blooded type and with 

 this change the Mammals first appeared. Similar transforma- 

 tion of a different group of Reptiles brought about the origin 

 of Birds to which rather than to Mammals all modern Reptiles 

 are related. The story of the Birds however lies beyond our 

 present scope. 



It is true that the organization of a Mammal differs very 

 greatly from that of a typical Reptile but in this account it is 

 unnecessary to deal in detail with the characters of the body as 

 a whole. We shall confine our attention to a few points illus- 

 trated by the dentition and the skull (Fig. 8). The mammalian 

 mandible is constituted by a single bone* the representative of 

 the reptilian dentary. The mandible in the Reptile on the 

 contrary is more complex and is formed by several elements. 



* Although the angular and surangular are not usually recognized many Mammals 

 show a mere rudiment of the former and Ornithorhynchus possesses vestiges probably 

 of both. 



