338 CHELONIA 



CHAP. 



of a Chelonian, its age would speak greatly in favour of the 

 Athecae being a very primitive and independent group. 



Order II. THECOPHORA. 



Thoracic vertebrae and ribs united with a series of median or 

 neural and a paired series of lateral or costal plates. Parietals 

 prolonged downwards, meeting the pterygoids directly or Inj inter- 

 position of an epipterygoid. 



Sub -Order 1. Cryptodira. The carapace is covered with 

 horny shields. The neck, if retractile, bends in an S-shaped curve 

 in a vertical plane. The pelvis is not fused with the she/I. 



Fam. 1. Chelydridae. The plastron is small and cross-shaped 

 (Fig. 61, 2, p. 315) ; the bridge is very narrow, and the displaced 

 abdominal shields, are widely separated from the marginals by a 

 few irregularly shaped inframarginals. The tail is long. The 

 limbs, neck, and head are so stout that they cannot be completely 

 withdrawn into the shell. Snout with a powerful hooked beak. 

 American ; only two genera, each with one species. 



The temporal region is roofed very incompletely and only 

 anteriorly by the expanded parietals and postfrontals, which form 

 a long suture. The plastron consists of nine bony plates, a small 

 entoplastron being present ; there are lacunae in the middle line, 

 the plates meeting imperfectly, and the horny abdominal shields 

 are likewise separated by soft skin. The carapace has a nuchal 

 with long rib-like processes which underlie the marginals ; the 

 neural plates form a continuous series. There are twenty-three 

 marginal plates. The pubic and ischiadic symphyses remain 

 separate, enclosing one large heart-shaped foramen. The five 

 fingers and toes are webbed and are protected by claws except the 

 outer toe, the nail of which is usually suppressed. 



Chelydra serpentina, the Snapping Turtle, attains a large size, 

 namely, a shell-length of more than one foot, and a total length 

 from the nose to the tip of the tail of more than three feet. 

 Its range extends from the Canadian lakes east of the Rocky 

 Mountains, through the United States and Central America. 

 The carapace of young specimens has three very marked series 

 of keels, which gradually disappear with age, until in very old 

 individuals the shell becomes quite smooth. The skin is very 

 warty, especially on the neck, and there is a pair of minute 



