334 ORDERS OF FISHES. 



CHAPTER XXX. 

 ORDERS OF FISHES Continued. 



ORDER III. ELASMOBRANCHII ( = Selachia, Muller ; PZacoidei, 

 Agassiz ; Holocephali and Plagiostomi, Owen). This order in- 

 cludes the Sharks, Rays, and Chimaerae, and corresponds with 

 the greater and most typical portion of the Chondropterygidce. or 

 Cartilaginous Fishes of Cuvier. The order is distinguished by 

 the following characters : The skull and lower jaw are well 

 developed, but there are no cranial bones ; and the skull consists of 

 a single cartilaginous box, without any indication of sutures. 

 The vertebral column is sometimes composed of distinct vertebra, 

 sometimes cartilaginous or sub-notochordal. The exoskeleton is in 

 the form of placoid granules, tubercles, or spines. There are two 

 pairs of fins, represe?iting the limbs, and supported by cartilaginous 

 Jin-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 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 of an osseous or cartilaginous disc, from the upper 

 surface of which springs a sharp recurved spine, composed of 

 dentine. 



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 gill-pouches open upon the sur- 

 face by a series of separate apertures, which are placed on the 

 sides of the neck in the former, and on the under surface of 

 the body in the latter. In neither is there any gill-cover or 

 operculum, nor are there any branchiostegal rays. In one 

 section of the order, however viz., the Holocephali though 

 the internal structure of the gills is essentially the same as in 



