SKELETON 



45 



AMPHIBIA. — Traces of a dermal skeleton are found in recent amphibians 

 only as scattered plates in the skin of the backs of a few exotic toads and as 

 rows of scales in the cutaneous rings of caecilians (fig. 36). These, which are 

 roughly circular in outline, bear small plates (squamulas) on the anterior (outer) 

 surface, these being roughly arranged in circles as if representing stages of growth . 

 Some fossil stegocephalans had an armor on the ventral surface while others 

 had one protecting the whole body. The elements of the ventral exoskeleton — 

 sometimes scales or plates, sometimes long bars — are arranged in oblique rows 

 (fig. 35) and possibly represent the source of the gastralia (p. 46) found in 

 several reptiles. Apparently certain of these ventral elements were modified 

 into comb-like organs which have been interpreted as sexual in function. Epi- 

 sternum and clavicle were possibly dermal in the stegocephals ; they are described 

 in connexion with the sternum (p. 66). 



^2t--. 



Fig. 36. — Section of a body ring of the Caecilian Epicrium showing the scales, after 

 the Sarasins. b, basal layer of the corium; c, gland in the corium; e, epidermis; g, 

 giant gland; s, scales, the black spots indicating the squamulae; v, vertical partitions of 

 the corium connecting the basal and superficial layers; the head toward the left. 



REPTILES. — Among living reptiles the dermal skeleton is best developed 

 in the turtles, though here it is closely associated with the endoskeleton. In 

 these animals the body is enclosed in a box, composed of dermal plates which in 

 part fuse with the endoskeleton. This box consists of a dorsal carapace and a 

 ventral plastron united at the sides to a varying extent, and each consisting 

 of a number of elements. In the middle line of the carapace is a series of neural 

 plates (fused below with the vertebrae) ; the margin of the carapace consists of 

 marginal plates, and between marginals and neurals are long costal plates fused 

 to the underlying ribs. The plastron (fig. 38) usually consists of nine wholly 

 dermal plates, their names shown in the figure. The three hinder pairs are 

 regarded as homologous with the gastralia of other reptiles, the anterior pair as 

 the clavicles, while the unpaired entoplastron is supposed to represent the epi- 

 sternum of other tetrapoda. 



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

 these are retained in a reduced condition in the living species. These reptiles 



