E lasmobran chii. 



81 



Sub-class I. ELASMOBRANCHII. 



The true fishes begin with the sub-class of Elasmobranchii 

 (laminated or plate-like gills). In these the cranium itself is 

 not divided into any distinct tracts by sutures or ossifications, 

 and the two foremost of the " visceral arches " (cartilaginous 

 rods in the walls of the alimentary tube), which are modified 

 as jaws and hyoid cartilages, have a very slight connection with 

 it. The jaws are mainly suspended by the upper element of 

 the hyoid arch (the " suspensorium") and by a ligament in 

 front ; or there is sometimes (e.g. in Cestracion and Notidanus) 

 direct contact either behind or in advance of the eye (see Fig. 118, 

 p. 88). The axial skeleton of the trunk varies from a primitive 

 persistent notochord to a well-calcified vertebral column, com- 

 posed of distinct centra. The gills are pouch-like, and there 

 are five (six or seven) distinct clefts on each side, which 

 are exposed, having no " gill-cover," or operculum. The body 

 is provided with median and paired fins, the hinder pair being 

 abdominal. 



In the majority of the Elasmobranchs, the extremity of the 

 vertebral column is slightly turned upwards, and the lower 

 lobe of the caudal fin is much larger than the upper, producing 

 a " heterocercal " tail. In some, however, like Squatina and 

 several of the Bays, the terminal portion of the body is straight, 

 and the fin equally developed above and below, upon the " diphy- 

 cercal " or " protocercal" plan. 



The skin is usually covered more or less closely by numerous 

 small detached plates or granules of dentine, with tubercles or 

 spines (Fig. 108) scattered over the whole surface of the integu- 

 ment, commonly known as " placoid scales." When very small 



FIG. 108. Dermal tubercles of Elasmobranch Fishes. 



a, shagreen of Dog-fish (Ginglymostoma), enlarged ; 6, shagreen of Blue Shark 

 (Carcharias), enlarged ; c, shagreen of Spiny Dog-fish (Centropliorus), enlarged ; 

 tZ, dermal tubercles of Spiny Shark (Echinorhinus), nat. size; e, tubercle of Ray; 

 /, dermal tubercle of Greenland Shark (Lcemargus borealis), enlarged; g, 

 'shagreen of Sting-ray (Urogymnvs), nat. size. 



(1876) 7 



Wall-cases, 

 Nos. 1 to 3, 

 and Table- 

 cases, N"os, 

 25 to 32. 



