vii.] THE EXTERNAL SKELETON. 263 



all Rodents, i.e. in all Rats. Squirrels, Hares, Marmots, and 

 Porcupines. 



Canines may grow from a root essentially similar to that 

 of man's canine, or from a double fang, as in Galeopithecus 

 and the Mole. On the other hand, as has been said, they 

 may grow from permanent pulps. 



Very rarely, e.g. in the Tanrec (Centetes), the points of the 

 lower canines are received into corresponding pits on the under 

 surface of the upper jaw. 



27. The MOLAR TEETH of man present a medium character 

 when compared with those of the other members of his class. 

 Often we find molar teeth relatively larger, broader, more 

 complex, and more suited for grinding hard or tough vege- 

 table substances than in him. 



We see, on the contrary, that they may often become 

 relatively smaller, narrower, and more trenchant, so as to 

 serve better for cutting and dividing flesh. 



The maximum both of size and complexity is attained by 

 the grinders of the Asiatic Elephant, where the worn surface 

 presents a great number of narrow transverse ridges, the nature 

 of which will be explained under the head of Structure. 



The greatest simplicity of form is seen in the Walrus, 

 where each molar is perfectly simple and flat ; and in the 

 blood-sucking Bat Desmodus, where each molar forms a 

 single wedge-shaped blade. 



The total number of molars reaches its maximum in the 

 existing creation in the little Marsupial Myrmecobius, where 

 they are thirty-four (f ) in number. The smallest number is 

 met with in the Australian Rat Hydromys, where we find |. 



28. The distinction between pre-molars and true molars 

 which exists in man, exists also in the great majority ot 

 animals belonging to his class. 



The number of PRE-MOLARS is often more numerous than 

 in him, there being three even in the American Apes, and four 

 in the Dog. 



In form the pre-molars are generally, as in man, smaller and 

 less complex than are the true molars ; but in some beasts, as 

 for example in the Horse, there is no difference in this respect. 



The first pre-molars soon present a striking difference from 

 those of man. Thus, even in the old-world Apes, the upper 

 one has its external cusp more prolonged, and the lower one 

 has its front edge elongated and blade-like. These teeth may 

 assume the shape of canines, as in Hemicentetes and in the 

 Camel. 



