109 



n*l lateral pieces have retained the name of ' marginal plat**.' Thr first or anterior of the 

 median plain (' nurhal plalc ') U remarkable for iu great breadth in the Turtles, and usually 

 toil down a ridge from the middle line of iu under surface, which articulates more or lest 

 directly with the summit of the neural arch of the first dorsal vertebra ; this may be seen in 

 the carapace of the TVwmyjr, No. 931 : the second neural plate is much narrower, and is 

 connate with the summit of the neural spine of the second dorsal Tcrtcbra : the seven suc- 

 ceeding neural plain have the same relations with the succeeding neural spines : the ret 

 are independent dermal bones, but the ninth is separated from the tenth by the hut pair 

 at costal plain. The costal plain of the carapace are superadditions to eight pairs of 

 the pleurapophyses or vertebral portions of the second to the ninth ribs inclusive. The 

 slender or proper portions of these ribs project freely for some distance beyond the con- 

 nate dermal portion*, along the under surface of which the rib may be traced, of its ordi- 

 nary breadth, to near the bead, which liberatn itself from the costal plate to articulate to 

 tin- interspace of the two contiguous vertebrae, to the posterior of which such rib pro|>erly 

 belongs. 



The plastron consists in the genus Ckeltmr, as in the rest of the Order, of nine pieces, 

 one median and symmetrical, and the rest in pain. With regard to the homology of these 

 bones, three explanations may be given : one in conformity with the structure of the- thoracic- 

 abdominal cage in the Crocodile ; the other based upon the analogy of that part in the Bird ; 

 and the third agreeably with the phenomena of development. According to the first, the 

 median piece of the plastron, called ' cnto-sternal,' answers to the sternum of the Crocodile, 

 or ' sternum proper,' and the four pain of plastron-pieces answer to the ' hsemspophy sea ' 

 ft M ir*"g the so-called sternal and abdominal ribs of the Crocodile. Mort Comparative Ana- 

 tomists have, however, adopted the views of Geoffrey St. Hflaire, who was guided in his deter- 

 mination of the pieces of the plastron by the analogy of the skeleton of the Bird ; according 

 to which all the parts of the plastron are referred to a complex and greatly developed ster- 

 num, and the marginal plain are viewed as sternal ribs (hsemapophyses). The third ground 

 of determination refers the parts of the plastron, like those of the carapace, to a combination 

 of parts of the endoskeleton with those of the exoskeleton. 



In the present skeleton the marginal plates are twenty-two in number, or twenty-four if 

 the first (nuchal) and but (pygal) vertebral plates be included. Omitting these in the enu- 

 meration, two marginal pieces intervene on each side at the angles between the first median 

 plate and the point of the first costal plale formed by the end of the second dorsal rib, which 

 point enters a depression in the third marginal piece ; the fourth, fifth, sixth, seventh, eighth 

 and ninth marginal plale* are similarly articulated by gomphosis to the six succeeding ribs ; 

 the tenth marginal plate has no conm|>oiiding rib ; the eleventh is articulated with the point 

 of the ninth dorsal rib supporting the eighth costal plate. 



The want of concordance with the vertebral ribs, or ' pleurapophyses,' arising from the in- 

 creased number of the marginal pieces, favours the idea of their being dermal ossifications, 

 such peripheral elements being more subject to vegetative division and multiplication than the 

 hsemapophysn : the absence of the marginal pieces in the Trionyi gives additional support 

 to the same view. The parial pieces of the plastron are the ' hssmapophyses ' connate with 

 expanded dermal ossiftcsrions, and have received the following special names : ' episternal.' 



Z 



