2402 



TORTOISES, TURTLES, AND PLESIOSAURS 



marked by a series of channels corresponding to the borders of these same shields. 

 If the shell be not that of a very aged animal, there will be seen in addition a num- 

 ber of finely jagged sutures, marking the divisions between the component bones; 

 and it will be noticed that in their plan of arrangement, although not in number, 

 size, or shape, these underlying bones correspond very closely with the overlying 

 horny shields. Thus, in the middle line of the carapace we have a series of polyg- 

 onal plates, symmetrical in themselves, and attached to the summits of the verte- 

 brae, which are known as neurals; these being clearly indicated in the figure referred 

 to. In front, the series is completed by a large nuchal plate, having no connection 

 with the backbone, while behind it terminates in one or two pygals, which are like- 



DIAGRAM OK THE HORNY PLATES ON A SHEW, OF A FRESH-WATER TORTOISE. 



(After Giinther.) 



wise perfectly distinct from the vertebrae. Externally to the neurals are placed on 

 either side the eight costal plates, so named from being attached to the ribs; the 

 inner halves of these plates being alone visible in the shell figured at the head of 

 the chapter, which belonged to a rather aged animal. Finally, the edges of the 

 carapace are formed by the marginal plates, which, like the horny shields similarly 

 named, are angulated, and form the lateral borders of the middle portion of the 

 plastron. In the plastron itself, we find its anterior portion formed by a pair of 

 plates, known as the epiplastrals , corresponding to the collar bones, or clavicles, of 

 other Vertebrates; while between or behind these is a single unpaired entoplastral 

 element, which may be either dagger shaped or rhomboidal, and which represents 

 the interclavicle of less specially modified reptiles. The remainder of the plastron 

 is formed by three pairs of plates, respectively known as the hyo, hypo, and xiphi- 

 plastrals, of which the latter or hindmost are generally more or less deeply notched 

 or forked. These three elements appear to correspond to the so-called abdominal 



