TEETH AND BONES 



vertebral column, there are two projecting condyles to 

 effect this articulation, whereas in reptiles there is but 

 one, although that one may show signs of division. The 

 practical effect of this is seen by the fact that a mammal 

 cannot twist its head to look directly backwards like a 

 bird can. The teeth of mammals generally (but here there 

 are exceptions, such as the whales) are divisible into 

 three distinct sets ; there are the small incisors in front, 

 the canines follow them, and the molar or grinding 

 teeth complete the series. The human jaw will illus- 

 trate this mammalian feature satisfactorily, though 

 the canines are in most persons not so marked off from 

 the last incisor in front of them and the first premolar 

 which follows. Still, many have "eye-teeth," as the 

 canines are sometimes termed, which are prominent 

 enough. Now in reptiles and amphibians there is 

 never so marked a differentiation into series as there is in 

 mammals. Furthermore (except again the whales) 

 the mammal has a limited number of teeth, the very 

 outside numbers being a trifle over fifty, as in certain 

 marsupials, while the reptile and the lower vertebrate 

 generally has frequently a large and almost indefinite 

 series. It has truly a mouth full of teeth. Some 

 reptiles, such as tortoises, and some amphibians, such as 

 toads of various kinds, are completely without teeth, 

 like the ant-eaters among mammals. But if teeth are 

 present, the characters just emphasized are usually to 

 be seen. These, however, are general characters ; 

 practically the zoologist does not consider them, for 

 the reason that he directly compares the given fossil 

 bone with the nearest thing in the bones of living 

 animals. For information of this kind the reader must 

 consult larger treatises, or still better, make himself 

 acquainted with the skeletons at museums. Even 

 then, with fragmentary remains which are so often it is 

 to be regretted the only vestiges of formerly existing 



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