ELASMOBRANCHII. 419 



sists of a single cartilaginous box, without any indication of 

 sutures. The vertebral column is sometimes composed of dis- 

 tinct vertebrae, sometimes cartilaginous or sub-notochordal. The 

 exoskeleton is in the form of placoid granules, tubercles, or 

 spines. There are two pairs of fins, representing the limbs, 

 and supported by cartilaginous fin-rays ; and the ventral fins 

 are placed far back near the anus. The pectoral arch has no 

 clavicle. The heart consists of a single auricle and ventricle, 

 and the bulbus arteriosus is rhythmically contractile, is pro- 

 vided with a special coat of striated muscular fibres, and is 

 furnished with several transverse rows of valves. The gills are 

 pouch-like. 



In most of the above characters it will be seen at once that 

 the Elasmobranchii agree with the Ganoid fishes, especially as 

 regards the structure of the heart. The following points of 

 difference, however, require more special notice : 



I. The exoskeleton is what is called by Agassiz "placoid." 

 It consists, namely, of no continuous covering of scales or 

 ganoid plates, but of more or less numerous detached grains, 

 tubercles, or spines, composed of bony matter, and scattered 

 here and there in the integument. In the case of the Rays, 

 these placoid ossifications often take a very singular shape, 

 consisting (fig. 161, c) of an osseous or cartilaginous disc, from 

 the upper surface of which springs a sharp recurved spine, 

 composed of dentine. 



II. The^y/j are fixed and pouch-like, and differ very mate- 

 rially from those of the Bony and Ganoid fishes. In the case 



Fig. 178 A, Head of Piked Dog-fish (Spina.r), showing the transverse mouth on 

 the under surface of the head, and the apertures of the gill-pouches. B, Diagram 

 of the structure of the gill-pouches ; o o External apertures ; i i Apertures leading 

 into the pharynx ; j j Gill-sacs, containing the fixed gills. 



of the Sharks and Rays, the structure of the gills is as follows : 

 The branchial arches are fixed, and the branchial laminae 

 are not only attached by their bases to the branchial arches, 

 but are also fixed by the whole of one margin to a series of 

 partitions, which divide the branchial chamber into a number 

 of distinct pouches (fig. 178). Each partition, therefore, car- 



