ANATOMY OF VERTEBRATES. 



61 



52 



The expanded thoracic-abdominal case, fig. 51, k, k, into 

 which, in most Chelonians, the head, the tail, and the four ex- 

 tremities can be withdrawn, and in some of the species be there 

 shut up by movable doors closely fitting both the anterior 

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

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

 and not the least interesting result 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-chamber of the Chelonia consists chiefly, 

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

 solely, of the floor and the roof: 

 side-walls of variable extent are 

 added in the fresh-water species 

 (Emys) and land-tortoises ( Tes- 

 tudo). The whole consists 

 chiefly of osseous ' plates ' with 

 superincumbent horny i scutes,' 

 except in Trionyx and Sphargis, 

 in which these latter are 

 wanting. 



The roof, or e carapace,' fig. 

 52, consists of a ' median ' series 

 of symmetrical plates, ch, s l to 

 s li, and of two 'lateral' series 

 forming a pair, pi l to pi 8, the 

 whole being surrounded by a 

 circle of ' marginal ' pieces, m l 



to py, completed anteriorly by ch, the first of the median 

 series. Of the median series eight, s l to s s, are attached to 

 the spines of eight subjacent vertebrae : the lateral or parial 

 plates, pi l to pi 8, are attached to, and more or less blended 

 with, the ribs of the same vertebra? ; and the ends of these ribs 

 usually articulate by gomphosis with a corresponding number 

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

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

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

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

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

 soft-turtles {Gymnopus, Sphargis). 



m io 



Carapace of the Loggerhead Turtle (Clielone 

 caouanna) 



