xv. 3 



CHELONIANS 



393 



The characteristic of chelonian organization is the shortening and 

 broadening of the body, together with the development of bony 

 plates, forming a box into which the head and limbs can be with- 

 drawn. The total number of segments is only about 8 in the neck, io 

 in the trunk, and a series of reduced caudals; the body is therefore 



Fig. 224. Skeleton of turtle (Chelone). 



carp, carpus (note hook-shaped 5th metacarpal); cen. centrum of vertebra; cor. coracoid; 

 fern, femur ;fib. fibula; h. humerus; il. ilium; isch. ischium; mar. marginal plate; nuch. nucal 

 plate; pr. 'proscapular' process or acromion; pub. pubis; rad. radius; rib, rib, partly fused 

 with costal shell-plate; sc. scapula (foreshortened); tib. tibia; uhi. ulna. (After Shipley and 



McBride and Reynolds.) 



morphologically shorter than in any other vertebrate except the frog. 

 Probably this shortening and broadening is the result of some quite 

 simple change in morphogenesis. 



The shell is usually considered to include a dorsal carapace and 

 ventral plastron. Each of these is made up of inner plates of bone, 

 covered by separate outer plates of horny material, comparable to the 

 scales of other reptiles. The carapace includes five rows of bony plates, 

 namely, median neurals, and paired costals and marginals (Fig. 225). 

 These plates are ossifications in the dermis, attached to the vertebrae 



