EXISTING ORDERS OF PAL&OSA URIA. $13 



leathery turtle Spharyis and the fossil Psepliophorus ; second, the 

 Peltochelyidae, including the Trionychidae, in which there are no 

 horny scutes, but a granular bony skeleton, like that of the Dermato- 

 chelyidre, is superimposed upon the bony skeleton of the carapace and 

 plastron ; third, the Aspedochelyidae, which comprises turtles, emy- 

 dians, and tortoises, distinguished by symmetrical horny scutes cover- 

 ing the bony carapace. 



We might expect the simple shield of Sphargis to be the oldest, 

 then theoretically it should precede Trionyx ; but Sphargis is only 

 known from the Upper Tertiary, and Trionyx dates back to the Creta- 

 ceous of the United States, The majority of Chelonian fossils belong 

 to the Emydian type. Among tortoises may be mentioned the gigantic 

 ColossodielySj of the Siwalik beds, and many species of Testiido. 

 Professor Cope records Pmtostega, the type of a family in which the 

 large dermal bones are separate from the ribs. Toxoclielys belongs to 

 the marine chelonia. In Europe, true marine chelonians are best 

 known from the Chalk and London Clay in .this country, though they 

 are said to date back to the Purbeck. The Emydians comprise, 

 among other types, Pelobatochelys and Enaliochelys from the Kimme- 

 ridge Clay, which also yields PlesiorJtelys, and, according to Sauvage, 

 Platemys, The Upper Jurassic rocks also yield Thalassemys, Tropid- 

 emys, CraspedocJielys, which have not been recognised in this country. 

 Eurysternon is characteristic of the Solenhofen slate, Tretosternon and 

 Pleurosternon characterise the Purbeck and Wealden beds, Chitra- 

 cepltalus is found in the Wealden of Belgium, which also yields Pelto- 

 chelys. Protemys is found in the Lower Greensand. Rhinochelys and 

 Trachydermochelys characterise the Cambridge Greensand. In the 

 Tertiary rocks of the Continent are found Chelydra, Trachaspis, 

 Aplwlidemys. Pulceochelys. There are many forms referred to Emys, 

 such as most of those which occur in the London Clay x and the Middle 

 Tertiary of the Continent. 



Crocodilia.^ The antecedent group from which a type has been 

 modified is often to be detected in the vertebral characters of the tail. 

 Thus in birds the caudal vertebras are often bi-concave, and a similar 

 character is found in the tails of crocodiles. Hence, just as birds have 

 existed with bi-concave vertebras, so also have crocodiles. The Croco- 

 dilia are hence often divided into two groups, viz., the Teleosauria, 

 comprising the genera of the older Secondary rocks with flat or concave 

 intervertebral articulation, and the Eucrocodilia, comprising the Cre- 

 taceous, Tertiary, and existing procoelous types. 



The teleosaurs 3 were preceded by Belodon or Phytosaurus of the 

 Trias. Tdeosaurus has a form of head like the gavial of the Ganges ; 

 but the posterior nares do not extend so far back. The genus has 

 been divided into several sub-genera. Mystriosaurus hast he skull flat- 

 and the eyes directed upward ; it is found in the Lias. Macrospon- 

 dylus only differs from Mystriosurus in the greater length of the ver- 



1 Palaeont. Soc. 



2 See W. K. Parker, F.R.S. : Trans. Zoological Soc., vol. xi., part 9, " On .Struc- 

 ture and Development of the Skull in Crocodilia." 



3 E. E. Deslongschamps : " Notes Palaeontologiques," 1863-1869. 

 VOL. I. 2 K 



