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 
birdcan. 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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