CHAPTER III 



THE ANCESTRY OF THE MAMMALIA 



Geological dawn of mammalian life — Differences between Rep- 

 tiles and Mammals — Origin of mammal-like Reptiles — 

 transformation of the limbs — development of the brain 

 in mammal-like Reptiles — Cynodonts — Protodonts — Multi- 

 tnbercnlates— Triconodonts — Tritubereulates — the parent 

 form of mammalian teeth. 



In the present volume the study of mammalian teeth is the 

 essential object. By comparison of tooth forms zoological rela- 

 tionships of the various Mammals and of Man will be indicated. 

 To institute comparisons of real value we must have a standard 

 and this standard will necessarily be the primitive type of 

 mammalian dentition. It is obviously futile to search for 

 parent or stem forms among the considerably specialized Mam- 

 mals existing today. Even among the remains of extinct 

 Mammals ancestral to and simpler in form than those now liv- 

 ing it is unlikely that the actual parent form* will be found 

 although the fossil Mammals of an earlier day may be expected 

 to show indications more or less clear of what that parent form 

 must have been. Our search for indications of the features of 

 this dawn type of Mammal must then be among the fossilized 

 faunal relics of earlier geological periods. 



With the exception of such rarities as the imprisonment of 

 entire Mammoths in the ice fields of Siberia and the preserva- 

 tion of portions of the skin of gigantic Ground Sloths in the 

 caves of Patagonia the only vestiges of extinct Mammals left 

 to us are teeth and bones. Scarcely ever is a skeleton found 

 complete and very frequently the component bones themselves 



*Presuming, as is probable, that all mammalian orders had a common ancestral 

 stem form. 



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