THEIR DIFFERENCES. 137 



reptiles known as dinosaurs, and ovight, therefore, to 

 be regarded witli respect as being the sole existing, 

 although collateral, representatives of that great grouj) of 

 reptiles which dominated the earth at a time when 

 mammals were only just beginning their career. 



Having said thus mucli as to the distinctness of croco- 

 diles from lizards, we may proceed to consider how the 

 former differ from alligators, and to make some mention 

 of a few of the various species of each. Now if we examine 

 the skulls of the different kinds of crocodiles we 

 shall find that the number of teeth in the upper jaw 

 varies from seventeen to nineteen, while in the lower 

 jaw there are invariably fifteen ; and we shall likewise 

 find that the teeth of the two jaws iuter]ocl< with 

 one another when the mouth is closed. Moreover, when 

 the jaws are in apposition it will be observed that 

 the first tooth on each side of the lower jaw is received 

 into a pit in the palate of the skull, while the fourth 

 lower tooth, which (like the first) is larger than the 

 others, bites into a notch in the side of the skull (as shown 

 in Fig. 44), and is thus more or less distinctly visible 

 externally in the living animal. Crocodiles are now found 

 in the rivers of Africa, India, Burma, Australia, and 

 America, as well as in many of the larger islands in warm 

 regions. They vary greatly in regard to the relative 

 length of the skull, the longest-snovited species occurring 

 in South America and West Africa, while those of India 

 have the shortest and broadest skulls (Fig. 45). On the 

 other hand, if we examine the skull of an alligator, we 

 shall find that the upper teeth bite on the outer side 

 of the lower ones without any sort of interlocking, and 

 both the first and the fourth lower teeth are received into 

 pits in the skull, so that when the mouth is closed l)oth of 

 them are totally invisible from the outer side. Moreover, 

 in no alligator does the number of lower teeth ever fall 

 short of seventeen. Again, in all alligators the skull is 

 even broader and shorter than in the Indian crocodiles. 

 Till within the last few years it was believed (in s]>ite of 

 the persistent assertion of sportsmen, that the Indian 

 "magars," as they are called by the natives, are alligators) 



