180 THE ANATOMY OF VERTEBRATED ANIMALS, 



but is quite soft. Tlie costal plates are shorter than in other 

 Chelonia, and the marginal ossicles are either rudimentary or 

 absent. 



The genera Gymnopus, Cryptopus, and Cydoderma, con- 

 stitute this division ; the}- all inhabit the fresh waters of hot 

 latitudes. 



The Euereta, or Turtles, have an exposed, hooked, horny 

 beak, with a blunt snout. The tympanum is hidden by the 

 integument. The limbs, of which the anterior pair are much 

 the longer, are converted into paddles, the digits being much 

 flattened and elongated, and immovably united together by 

 the integument ; only one or .two nails are developed. The 

 skin of the body is either rugose {Sphargis), or covered with 

 thick epidermic plates ( Chelone.) 



The two genera composing this group inhabit the seas of 

 warm climates. 



The Chelonia are first known to occur, with certainty, in 

 the Lias. The older forms are, in many respects, intermediate 

 between the Euereta and the Trionychoidea, but present no 

 approximation to any other order of Reptilia. 



II. The Plesiosaueia. — In some of the Plesiosauria, the 

 head, not more than one-twelfth or one-thirteenth of the length 

 of the bodv, is mounted upon a neck as long, in proportion, as 

 that of a Swan ; but in others, the head is large and massive, 

 and the neck much shorter. The hind-limbs are longer than 

 the fore-limbs, and there is a comparatively short tail. The 

 integument was certainly devoid of any scutes ; and was, prob- 

 ablj', smooth and possessed no scales. 



The cervical vertebrae maj' exceed forty in number, though 

 they are generally fewer ; and as none of the ribs appear to 

 have been connected with the sternum, or if such connection 

 existed it cannot now be traced, it becomes difficult to dis- 

 tinguish between cervical and dorsal vertebrse, and one is 

 obliged to have recourse to some method of separating the 

 two, diifering from that already adopted. Now, in these 

 animals, the neurooentral suture persists for a considerable 

 period, if not throughout life ; and the surfaces for the articu- 

 lation of the cervical ribs, which are at first altogether below 

 the neurocentral sutures, gradually rise, in the posterior parts 

 of the neck, until they first are cut by, and then rise above, 

 the suture. It is very convenient, and harmonizes very well 

 witl , some facts to be mentioned by-and-by in the structure of 

 the Crocodilia, to take the last of the vertebrae in which the 



