CROCODILES. 329 



Eastern United States, and India, and Parasuchus, from India. In 

 the deposits of the succeeding Jurassic age the number of distinct 

 types and species is very largely increased. No less than forty 

 species, belonging in the main to the genera Mystriosaurus, Teleo- 

 saurus, Steneosaurus, Metriorhynchus, and Dakosaurus, are known 

 from British strata alone."' Many of these are also found in the 

 deposits of the continent of Europe, which comprise a considerable 

 number of additional types. The amphicoelous, or biconcave, type 

 of vertebra, distinctive of the Triassic and Jurassic crocodilians, is 

 retained in a measure by the Cretaceous forms, as in Goniopholis 

 and the American Hyposaums,* but we now also meet, and for the 

 first time, with the type of the modern procoelian crocodile. Gavialis 

 and Crocodilus, abundantly developed as Tertiary forms, both oc- 

 cur in the Upper Cretaceous beds of Europe, and are represented 

 in the nearly equivalent American deposits by the gavialine genera 

 Holops and Thoracosaurus. Tomistoma is found in the Miocene 

 of Malta and Lower Austria. Alligator does not appear before tlie 

 Tertiary (Eocene) period (Europe and America). A gavialine form 

 from the Siwalik deposits of India, Rhamphosuchus crassidens, is 

 supposed to have attained a length of from fifty to sixty feet. 



The origin of the crocodilian line is involved in much obscurity. 

 Whether or not the animals of this group stand in direct genetic 

 relation with some of the earlier labyrinthodonts, as is maintained 

 by some paleontologists, our present knowledge does not permit us 

 to determine. Among themselves, however, the different croco- 

 dilian types exhibit a remarkable gradational series of structural 

 peculiarities, which connect the moat ancient and the modern forms, 

 and place them in an almost unbroken sequence. Professor Huxley 

 has indicated the line of succession as passing from the Parasuchia 

 — the Triassic forms, in which neither the palatine nor pterygoid 

 bones enter into the formation of secondary posterior nares — through 

 the Jurassic and Cretaceous Mesosuchia, in which the palatines 

 alone are produced to form these nares, to the modern and Upper 

 Cretaceous (procoelian) Eusuchia, in which both bones are similarly 

 produced. M. DoUo recognises in Bernissartia, a recently discov- 



* Hyposnurus Eogersi, from tho " {ciccnsands " of the Eiuitem United 

 States, Wits until recently the only known species of Oie genus; a second 

 species, II. Dorbianus, lias been described by Professor Cope from the Province 

 of Pernambuco, Brazil ("Trans. Am. Phil. Soo.," Jan., 1888). 



