44 DISTRIBUTION OF CROCODILES. [CH. I 



species, A. sinensis, was made known in 1879. The 

 Caimans and Perosuchus are exclusively tropical American. 

 Ostolcemus is West African. Oavialis is confined to some 

 of the rivers of India, while Tomistoma has only been met 

 with in Borneo. 



Though the two latter genera by their elongated 

 snouts suggest the Mesozoic Teleosaurians, it seems 

 probable that Caiman and Perosuchus represent the 

 most archaic among the existing Crocodilia. The reason 

 for this opinion is that they alone possess a ventral as well 

 as a dorsal armature of scutes, such as were developed in 

 forms like the Wealden Bemissartia, in which the ancestor 

 of both Crocodiles and Alligators is seen by some. To a 

 feeble extent the ventral scutes are to be found in 

 Alligator, and also apparently in Ostolcemus. This, it 

 will be noticed, is quite in accord with the wide but 

 discontinuous distribution of those genera, which might 

 almost on. this account be put into a separate family. The 

 presumption would be that formerly they were more 

 widely spread, but that the process of time produced gaps 

 in their ranks, leaving the present detached fragments. 

 The existence therefore of an Alligator in China is not so 

 remarkable if this point of view be borne in mind. 



As to the true Crocodiles of the genus Crocodilus, they 

 are characterised also by the fact that there are constantly 

 fifteen teeth only in the lower jaw; the old idea that a 

 Crocodile could always be distinguished from an Alligator 

 by the fact that the fourth tooth in the lower jaw was 

 received into a notch instead of into a pit in the upper 

 jaw has been exploded by the discovery that in an 



