244 



NATURAL HISTORY. 



A view of the carapace from above, when the ornamental part of the shell is removed, shows 

 a row of expanded spinous processes in the middle line, flanked by the expanded costal plates 

 which cover in the greater part of the carapace. At the edges of each of these is a projection 

 narrower than they are, and it is the outward continuation of the rib. The ends of the ribs are fixed 

 into a rim of marginal plates. There is a broad' plate in the middle line (the nuchal) in front, 

 connected on each side with the front marginal plate ; and in some kinds, behind, there is a 

 pygal plate in the middle line, and, like the nuchal, it is attached to the series of expanded 

 spinous processes. 



The under part of the buckler, or " the plastron," is not an outside development of the breast- 

 bone or sternum, but is formed exclusively by skin or dermal bones, and usually consists of nine 



pieces or plates, more or less developed. 

 Of these, one piece is in front, and there 

 are four pieces on each side, between 

 which there may or may not be a 

 middle space, which is closed by skin or 

 by cartilage. 



These different plates have received 

 names which formerly were derived from 

 the belief that this bone expansion was 

 a breast- bone or sternum modified. Now 

 it being believed that these bony pieces 

 are all ossifications of the skin, or are 

 what is called membrane bones, the names 

 are different, and relate to the word 

 plastron and their relative position. 



It has been noticed that the spaces 

 left between the plates of the plastron 

 are filled up with cartilage in some 

 Chelonians, and with skin in others ; and 

 it is readily observed that in the cara- 

 pace of the Green Turtle, which has been 

 taken as the example of Chelonian 

 upper armour, the vacant spaces between 

 the ribs and the marginal plates are 

 also closed by cartilages. 



Some of the Chelonians have enor- 

 mous shells, and in others they are small 



in relation to the size of the body ; in some the texture of carapace and plastron is all bony, and 

 in others it is gristly or cartilaginous, or bony, with a softer margin. The shells of some are flat, as 

 in the Turtles; in others, as in the great Tortoises, the carapace is high and arched. Many 

 Chelonians cannot retire the head, tail, and limbs within the protection of their shells, and others 

 can do this perfectly. Most have the upper and lower parts of their more or less protecting cases 

 soldered together at the edges, but in some there is only a cartilaginous or gristly union; and whilst 

 in the majority the carapace and plastron are immovable in themselves, in some there is an amount 

 of mobility in the plastron, by its having one or more unossified and cartilaginous parts stretching 

 across from one side to the other. 



The carapace and plastron may be beautified by an outside covering, the " tortoiseshell," which 

 differs greatly in its kind and ornamentation in the various groups into which the Chelonia are 

 divisible. In many the well-known " tortoiseshell " covers over all the hind parts, and in some only 

 a portion of the protecting case is thus ornamented. There are some Chelonians with a covering 

 resembling leathern skins, and others have the plastron covered with soft skin. The " tortoiseshell," 

 which is the ornamental horny covering to the carapace and plastron, and out of which hair-combs, 

 knife-handles, and inlaid work are made, is to all intents and purposes an outside or scarf-skin 



SKELETON AND CARAPACE OF THE LOGGER-HEADED TURTLE, 

 FEOM BELOW. 



