IX 



CARAPACE 



321 



and the first metacarpal are enlarged and flattened, thereby 

 giving additional width to the paddle. The tarsus remains less 

 primitive ; the centrale and the proximal elements have a 

 tendency to fuse together, most com- 

 pletely in land- tortoises ; the fifth 

 distal carpal is enlarged, and stands 

 out hook-like from the rest. The 

 number of the phalanges of the 

 fingers and toes varies slightly. It 

 is noteworthy that none of the 

 Chelonia possess more than three 

 phalanges. The three middle fingers 

 and toes have mostly three phal- 

 anges ; the pollex and hallux have 

 always two ; the number of phal- 

 anges of the fifth finger varies from 

 three to one, of the fifth toe from 

 two to none. 'The greatest reduc- 

 tion occurs in Testudo and its allied 

 genera of typical land - tortoises, 

 Homopus, Pyxis, and Cinixys, the 

 formula for the fingers being 

 2, 2, 2, 2, 2 or 1, and 2, 2, 2, 2, 

 for the toes. In Pelomedusa all the fingers possess two phalanges 

 only, owing to fusion of the first and second phalanges with each 

 other. 



The shell, which is the most characteristic feature of the 

 Chelonia, consists of the dorsal " carapace " and the ventral 

 " plastron." Each is composed of a considerable number of bony 

 plates which arise as ossifications of nearly the whole thickness 

 of the cutis, only a thin layer of subcutaneous connective tissue 

 remaining soft and lining the inside of the shell. We restrict 

 ourselves to a description of the shell of the Thecophora, leaving 

 the discussion of the peculiar shell of Sphargis to p. 336 f. Very 

 young tortoises are still soft, and the plates which are beginning 

 to ossify are not yet suturally united. The plastron (Figs. 6 6 and 

 6*7) consists of the paired epi-, hyo-, hypo-, and xiphi-plastral 

 plates, and the unpaired endo-plastral plate. The latter is homo- 



1 It should be noted that the horny pieces of the carapace are termed " shields '* 

 and the bony pieces "plates." 



VOL. VIII Y 



FIG. 66. The bones composing the 

 plastron of Chelone mydas. On 

 the right side the position of the 

 covering horny shields 1 is indi- 

 cated by dotted lines, a, Anal 

 horny shield ; ab, abdominal ; /, 

 femoral ; g, gular ; h, humeral ; 

 ig, intergular ; im, infra-marginals ; 

 p, pectoral. 



