SKELETON. 41 



each scale truncate and this edge and the surface toothed. Cycloid and ctenoid 

 scales intergrade and both kinds may occur on the same fish (many gobiids). 



AMPHIBIA. A dermal skeleton occurs in the recent amphibians only as rows 

 of plates in the cutaneous rings on the bodies of the caecilians and in the skin of the 

 back of a few exotic toads. In some fossil stegocephalans there was a ventral 

 armor and in others one protecting the whole body. The ventral exoskeieton, 

 sometimes of scales or plates, sometimes long bars, is arranged in oblique rows, 

 and is interesting as probably being the source of the gastralia found in many 

 reptiles (infra). Episternum and clavicle were possibly dermal in these forms, 

 but they will be described in connection with the shoulder girdle. Apparently 

 certain of the gastralia of these fossils were modified into comb-like organs which 

 have been thought to have sexual significance. 



REPTILES. The dermal skeleton is best developed in the turtles of living 

 reptiles, though here it is closely associated with the endoskeleton. The dermal 

 plates form a box for the protection of the body. This consists of a dorsal carapace 

 and a ventral plastron, united to varying extents and each consisting of a number 

 of elements. In the carapace there is a middle line of neural plates (fused with 



m 



FIG. 33. Section through developing vertebra, rib and exoskeieton oiChelone imbricata, 

 after Gotte. c, cutis; cs, primitive vertebral body, ep, epidermis; m, external oblique muscle; 

 p, perichondrium; r, rib; sp, spinal process. 



the vertebrae), marginal plates around the margin, and costal plates, fused to 

 the ribs, between neurals and marginals. The plastron (fig. 34) usually consists 

 of nine plates, wholly dermal, the names shown in the figure. The three posterior 

 pairs are regarded as the same as the gastralia of other reptiles, the anterior pair as 

 the clavicles, while the unpaired entoplastron is supposed to be homologous with 

 the episternum of other tetrapoda. 



Some of the extinct crocodilia were armored with closely applied scales and 

 these have been retained in the existing species in a reduced condition. They 

 also have well developed gastralia. These are of rods dermal bone in the ventral 

 body wall between the true ribs and the pelvis, and so closely resemble ribs that 

 they were called 'abdominal ribs.' They do not meet in the middle line; each, 

 except the first, consists of two distinct parts, and the pairs correspond to the 

 somites in number. In Sphenodon (fig. 35) the gastralia are more numerous than 

 the somites. 



In a few lizards there are dermal scales, while the extinct stegosaurs had 



