VERTEBRATA: FISHES. 495 



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 provided 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 or dentinal 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. 247, c) of an osseous or cartilaginous 

 disc, from the upper surface of which springs a sharp recurved 

 spine, composed of dentine. The so-called " shagreen " of the 

 Dog-fishes and Sharks is composed of very small and close-set 

 tooth-like processes (fig. 247, d}. At other times the placoid 

 structures are developed into "dermal defences" or "ichthyo- 

 dorulites." The minute structure of these exoskeletal struc- 

 tures is closely or entirely similar to that of the teeth. In some 

 cases the exoskeleton is absent. 



II. The gills are fixed and pouch-like, and differ very mate- 

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

 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. 268). Each partition, therefore, car- 

 ries a series of branchial laminae attached to each side, like 

 the leaves of a book. By means of these septa a series of 

 branchial sacs or pouches are formed, each of which opens 

 internally into the pharynx by a separate slit, and communi- 

 cates externally with the water by a separate aperture placed 

 on the side of the neck (fig. 268, B). The arrangement of the 

 gills being such, there is, of course, no gill-cover, and no bran- 

 chiostegal membrane or rays. In one section of the order, 

 however viz., the Holocephali though the internal structure 



