THE CARAPACE OF THE TORTOISE. 



243 



order may be divided into corresponding families, under the separate titles of Testudines, Emydes, 

 Trionycides, and Cheloniades. The difference of shape and construction of the members of these 

 t; families " is clearly connected with their diverse methods of living. The kinds of some families 

 swim freely ; those' of others swim and walk, or walk without being able to swim. Moreover, 

 there is a connection between their habits and the food, some being carnivorous, others enjoying a 

 mixed diet, and the rest being vegetarians. All lay eggs, and leave them to Dame Nature's care, and 

 hibernation is common in all except the marine kinds. 



The buckler of the Chelonians consists of a deeply-seated bony framework, the top of which, 

 situated on the back of the animal, is called the carapace, and the underneath, placed below the 

 body, is termed the plastron, or by some 

 the sternum. There is in many Che- 

 lonians a shell which covers these parts, 

 and is ornamented ; and in one species 

 this shell is the " tortoiseshell " of 

 commerce. In others, however, it is 

 not found, and its place is occupied by 

 skin. Curious as these protecting buck- 

 lers are, they are still formed by striic- 

 tures which can be traced in most 

 vertebrate animals, but which have been 

 modified to suit the reqiiirements of the 

 Chelonians. The carapace on the top of 

 the Chelonian is a modification of the 

 bones of the vertebral column and ribs, 

 and also consists of bony plates which, 

 growing in the skin, unite all together. 



The spinous processes of the dorsal 

 vertebrae and their ribs enter into the 

 composition of the framework of the 

 carapace, and a certain number of single 

 and of paired pieces of dermal bones 

 complete it. These dermal bones are 

 called complementary plates, and single ones may be in the neck, in the middle line, or behind 

 in the sacral region, whilst there are twenty-two plates on the sides, eleven being on each side. 

 These accessory skin bones are called respectively cervical, sacral, or pyyal, and marginal plates. 



A careful examination of the inside of a carapace shows that the spinous processes of the second 

 to the eighth dorsal vertebrae are flattened out, so as to form the middle of the buckler and compose 

 the median plates. Then it appears that the ribs of these same vertebra?, and of the ninth also, are 

 enlarged above so as to form eight long flat plates (costal"), which are united with those in front 

 and behind by toothed joints or interlocking sutures. 



The expanded ribs also unite with the median plates, formed by the expanded spinous 

 processes ; and a portion of each rib may be seen underneath the plate-like pai-t and projecting 

 beyond it at the side of the carapace, and its outer or free end is received into a pit in one of the 

 marginal plates. 



But as the expansion of the rib passes to the spinous process to form part of the carapace, 

 it is separated from the narrower part of the rib, which comes as usual from the side of the vertebrae. 

 A space thus exists between the ribs and their expansions, and it contains the muscles of the back. 



The under view of the carapace shows the bones of the dorsal vertebrae, which enter into 

 its composition in the middle line ; and on each side are the ribs, with their expanded portions 

 filling up the spaces between them. A transverse section of the carapace shows that the breadth 

 is greater than the height, as a rule, and that an upper arch is formed by the ribs and their ex- 

 pansions, and the expanded spinous processes of the vertebrte form the key-stone. The sides of the 

 plastron are seen as curved lines ; and at the sides is a marginal plate, connecting the plastron and rib. 



SKELETON OF TORTOISE, FROM JiELOW, SHOWING INSIDE VIEW OF 

 CARAPACE AND PLASTRON. 



