CHELONIA. 



31 



the others are free ; the first, ch, is termed the " nuchal," and 

 the last, py, the " pygal " plate. The contiguous side-plates, 

 pi 1 to pi 8 are dermal plates, connate with the eight subjacent 

 ribs, whence they are called "costal plates." External to 



Fig. 108. 

 A, Carapace. B, Plastron. 



Of a turtle, Chelone imbricata. 



these are the "marginal plates," m 1 to m 12, which are 

 wholly dermal ossifications, are inconstant in number, and 

 are wanting in mud-turtles. 



The plastron (fig. 108, b) consists of the sternum s, and of 

 four pairs of sternal ribs, with some or all of which dermal 

 bones of diverse forms and extent are connate. These com- 

 posite plates are of determinate number in existing Chelonia, 

 and have received special names. The single median piece, 

 s, is the " entosternal," the foremost of the parial plates, es, 

 are the " episternals," the next hs the " hyosternals," ps marks 

 the " hyposternals," and xs the " xiphisternals." 



Each of the above defined elements of the carapace and 

 plastron may shew characters indicative of the nature and 

 affinities of the chelonian it helped to house. The floor and 



