THE EXOSKELETON. 43 



place in laminse, of whicli the oldest is the most superficial, 

 and lies immediately beneath the epidermis. As a general 

 riile, the calcified tissue of the " scale " thus formed, does 

 not possess the structure of true bone in the Teleostei. But 

 in other fishes, the dermal calcification may consist of true 

 bone (as ia the Sturgeon) ; or, as in the Sharks and Rays, 

 may take on the structure of teeth, and consist maialy of a 

 tissue exactly comparable to dentine, capped with enamel, 

 and continuous by its base with a mass of true bone, which 

 takes the place of the crusta petrosa, or cement of the teeth. 

 A form of dermal esoskeleton, which is pec\tliar to and 

 highly characteristic of fishes, is f oxmd in the fin-^ays. These 

 are developed in the integument either of the median line of 

 the body, or in that of the limbs. In the former case, they 

 usually enter into, or support, folds of the iategument which 

 are termed dorsal, caudal, or anal fins — according as they 

 lie in the dorsal region, or at the extremity of the body, or 

 on the ventral aspect, behind the anvis. Ordinary fin-rays 

 are composed of a hornlike, or more or less calcified, sub- 

 stance, and are simple at the base, but become jointed trans- 

 versely, and split up longitudinally, towards their extremities 

 (Fig. 6). Each fin-ray consists of two nearly equal and simi- 

 lar parts, which cohere by their applied faces for the greater 

 part of their extent ; but, at the base of the ray, the halves 

 commonly diverge, to embrace, or more or less completely 

 coalesce with, cartilaginous or osseous elements of the exo- 

 skeleton. In the median fins, these are the interspinous 

 cartilages, or bones, which lie between the fin-rays and the 

 superior or inferior spines of the vei-tebrse. In the paired 

 fins, they are radial or basal, cartUagiaous or osseous, 

 elements of the endoskeleton. 



The Amphibia in general are devoid of dermal exoskeleton, 

 but the Ccecilice have scales like those of fishes. Ceratophrys 

 has plates of bone developed in the dorsal integument, 

 which seem to foreshadow the plates of the carapace of 

 the Chelonia ; and the extinct Labyrinthodonts possessed a 

 very remarkable ventral exoskeleton. 



The Opliidia have no denual exoskeleton. Many Lizards 



