CROCODILIA 207 



of considerable size, though none known was as large as some of 

 the late crocodiles. This type, with biconcave vertebrae, con- 

 tinued to live on, in both North and South America, to the latter 

 part of Cretaceous times, and it is even possible that some con- 

 tinued on into the Tertiary. But long before the close of the Cre- 

 taceous, the modern kind appeared, those with concavo-convex 

 vertebrae, and more posterior internal nostrils. The earliest are 

 known from New Jersey {Thoracosaurus, Holops), so like the Borneo 

 gavials of today that they are properly classified in the same family, 

 the Tomistomidae or GaviaUdae. If all the later, procoelian type, 

 that is, those with concavo-convex vertebrae, originated from a 

 single form when the amphicoelian or mesosuchian type became 

 extinct, Huxley's classification into the Mesosuchia and Eusuchia 

 would perhaps be proper, but we have much reason to suppose 

 that the change in the kinds of vertebra and in the position of 

 the nostrils was only incidental, and may have occurred in more 

 than one line of descent, that is, it may have occurred in the broad- 

 headed kinds of the Jurassic to the broad-headed crocodiles of today, 

 as also in the gavial-hke forms of the Cretaceous to the gavials of 

 the present.. And this is the reason why naturalists no longer 

 recognize the classification of Huxley, which, partly perhaps because 

 of the prestige of his name, has so long been accepted in our chief 

 works on natural history. 



MARINE CROCODILES, THALATTOSUCHIA 



While the ancient crocodiles of which we have spoken resembled 

 the modern ones so closely in form of body and probably in habits, 

 there were certain others of the old Jurassic seas which departed 

 so widely both in structure and in habits, from their associates that 

 they are by some authors given a place whoUy by themselves 

 as a distinct group. This has been called by Professor Fraas 

 the Thalattosuchia, a word meaning "sea-crocodiles." They were 

 a very early side-branch from the great genealogical tree of the 

 Crocodilia, a branch which departed so widely from their asso- 

 ciates in adapting themselves to a peculiar and aberrant mode of 

 existence that they cannot be considered as typical crocodiles, 

 although so closely related to them in other respects that there 



