AND ON THE EVOLUTION OE THE CTtOCODTLIA . 431 



of New Jersey* by Thoracosaurus (Leidy), Holops (Cope), and 

 other forms, and the Mesosuchia by Hyposaurus (Owen). On this 

 side of the Atlantic the Gavialis macrorhynchus of the " Calcaire 

 pisolithique " of Mont Arine (Upper Cretaceous) and the Maestricht 

 Chalk f is a completely differentiated Eusuchian. 



It is interesting to observe that Thoracosaurus has large praela- 

 chrymal vacuities like a Teleosaurian, and that the opening of the 

 median Eustachian canal is " a large transversely oval pit " (Leidy, 

 I. c. p. 7). The subvertebral processes of the cervical vertebrae are 

 small and divided. Leidy figures a biconvex first caudal vertebra, 

 though it is not certain that it belonged to this genus. I am inclined 

 to suspect that the scute of Thoracosaurus, " without any trace of 

 carina " (I. c. p. 11), indicates ventral armour. 



Hyposaurus had amphiccelous vertebrae ; but Leidy states that the 

 anterior concavities of these vertebrae are more deeply depressed than 

 the posterior (I. c. p. 20). 



On the other hand, Holops and Gavialis macrorhynchus appear 

 to differ very little from existing Crocodilia ; so that, in the case of 

 the Crocodilia, as in that of many other groups, the real " dawn " 

 of the existing fauna is not in the Eocene, but in the latter part of 

 the Cretaceous formation. 



Thus the facts relating to the modifications which the Crocodilian 

 type has undergone since its earliest known appearance, are exactly 

 accordant with what is required by the theory of evolution ; and the 

 case of the Crocodiles is as cogent evidence of the actual occur- 

 rence of evolution as that of the Horses. 



It must be understood that in maintaining that the Crocodilia 

 were at first Parasuchia, then Mesosuchia, and lastly Eusuchia, I do 

 not suggest that the progression has been effected through the 

 forms with which we happen to be acquainted. At the present 

 day there are two extreme terms of the Crocodilian series, the 

 Gavials being modified in accordance with the needs of an almost 

 exclusively aquatic life, and the Crocodiles and Caimans being much 

 more terrestrial in their habits. It is probable that this distinction 

 has been maintained, and that there have been more aquatic and 

 less aquatic forms throughout the greater part of the existence of 

 the Crocodilia. 



Unfortunately the only middle Mesozoic Crocodilia with which 

 we are well acquainted are the marine, probably sublittoral forms of 

 Mesosuchia, such as Teleosaurus and Steneosaurus ; while the older 

 Mesozoic Crocodilia are the probably freshwater Parasuchia, of a 

 more terrestrial habit. 



Thus, before being able to construct the complete ancestral tree of 

 the Crocodilia, we need to know those forms (if such existed) of Para- 

 suchia which were more aquatic than Belodon and Stagonolep)is — 

 and, on the other hand, those forms of Mesosuchia which were less 



* Leidy, " Cretaceous Eeptiles of the United States," Smithsonian Contribu- 

 tions, xiv. 1865. Cope, " Synopsis of Extinct Batrachia and Eeptilia," Trans- 

 actions of the American Philosophical Society, 1869. 



t Gervais, ' Zoologie et Paleontologie Francaises,' ed. 1859, pp. 431-457. 

 Q. J. G. S. No. 123. 2 g 



