444 CHORDATA. 
body is enveloped in this hard protective case, which is 
formed of a dorsal carapace and a ventral plastron, joined 
together laterally. The carapace usually has a median row 
of neural plates resting over the neural region, a lateral row 
of costal plates modified from the ribs, and a distal row of 
marginal plates. The plastron seems to be made up of a 
Fig. 314.—SKELETON OF A TORTOISE. 
The plastron has been cut away on the right side of the body and thrown over 
to the animal’s left. Note the carapace and plastron, flexible cervical and caudal, 
but ankylosed dorsal, vertebrae. 
pair of c/avicles and an episternum anteriorly and a num- 
ber of abdominal ribs posteriorly. The “case” of the 
Chelonia is evidently to a large extent composed of a number 
of pre-existing structures. Inside the case the vertebra, as 
might be supposed, are vestigial, with the exception of the 
cervicals and caudals. All four limbs are present and 
