208 THE ANATOMY OP VERTEBBATED ANIMALS. 



the pelvis is free, the neck bends in a vertical plane, and the 

 head is almost completely hidden by the carapace when 

 retracted {Envijs, Cistudo, Chehjdra). In Cistudo, Cinoste- 

 nuin, and Stawotypus, the hinder part of the plastron is 

 mobile. In the other division, the Chelodines, the pelvis 

 is fixed to the carapace and plastron, the neck bends side- 

 ways, and the head cannot be completely retracted under 

 the carapace (Chelys, Chelodina). 



3. In the Trionychoidea (Mud or Soft Tortoises), the jaws 

 have an external cutaneous lip; the nasal organ is pro- 

 longed into a kind of snout, and the head is covered by a 

 soft skin without any visible tympanic membrane. The 

 limbs are flattened, somewhat finlike, and pentadactyle ; 

 but only three digits have nails. The integument developes 

 no homy plates, but is quite soft. The costal plates are 

 shorter than in other Chelonia, and the marginal ossicles 

 are either rudimentary or absent. 



The genera Gymnopus, Cryptopus, and Cyclodernia, con- 

 stitute this division ; they 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 paii* are 

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

 much flattened and elongated, and immoveably 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 fij'st kno^vn to occur, with certainty, in 

 the Lias. The older forms are, in many respects, interme- 

 diate 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 body, is mounted upon a neck as long, in 



