ANATOMY OF VERTEBRATES. 



61 



The expanded tlioracic-abdominal case, fig. 51, K, K, into 

 wliicli, in most Clielonians, tlie head, the tail, and the four ex- 

 tremities can be withdrawn, and in some of the species be there 

 sliut np by mo^-alile doors closely fitting both the anterior 

 and posterior apertures — as, e.g., in the box-tortoises (Cino- 

 steriioii, Cistudo) — has been the subject of many investigations; 

 and not the least interesting residt has been the discovery 

 that this seemingly special and anomalous superaddition to the 

 ordinary vertebrate structure is due, in a great degree to the 

 modification of form and size, and, in a less degree, to a 

 change of relative position, of ordinary elements of the vertebrate 

 skeleton. 



The natural dwelling-chamlaer of the Chdonia consists chiefly, 

 and in the marine species ( Cheluue) and soft-turtles ( Trionyx) 

 solely, of the floor and the roof: 

 side-walls of variable extent are 

 added in the fresh-water species 

 {Eiiu/s) and land-tortoises ( Tes- 

 tudo). The whole consists 

 chiefly of osseous ' plates ' with 

 superincumbent horny ' scutes,' 

 except in Trionyx and Spharyis, 

 in which these latter are 

 wanting. 



The roof, or ' carapace,' fig. 

 52, consists of a ' median ' series 

 of symmetrical plates, ch, s i to 

 6-11, and of two 'lateral' series 

 forming a pair, -pi i to pi 8, the 

 whole being surrounded by a 

 circle of ' marginal ' pieces, m i 



to py, completed anteriorly by c/*, the first of the median 

 series. Of the median series eight, s i to « 8, are attached to 

 the spines of eight subjacent vertebra; : the lateral or parial 

 plates, pd 1 to p)l 8, are attached to, and more or less blended 

 with, the ribs of the same vertebraj ; and the ends of these ribs 

 usually articulate by gomphosis with a corresponding nvnnber 

 of the marginal pieces, of which, however, there may be from 

 twenty-four to tAventy-six, including the two median and symme- 

 trical ones, di and py. That these marginal pieces are the least 

 essential parts of the carapace is shown, not only by their incon- 

 stant number, Isut by their partial or total absence in some of the 

 soft-turtles {Gymnopus, Sphargis). 



m.7 



Carapace oC tiio Lnggcrbond Tm-tlc iChdouG 

 caouaiiiui) 



