GENERAL CHARACTERISTICS. 



45 



as shown in the right-hand figure at the head of the chapter, will be seen to be 

 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 

 number 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 

 ov 7 erlying horny shields. Thus, in the middle line of the carapace we have a series 

 of polygonal plates, symmetrical in themselves, and attached to the summits of the 

 vertebras, 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, 



DIAGRAM OF THE HORNY PLATES OX A SHELL OF A FRESH- WATER TORTOISE. After Giinther. 



which are likewise 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 xiphiplastrals, 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 ribs of crocodiles ; and it will thus be evident that Chelonians have 



