240 ZOOLOGY. 



The genuine cartilaginous fishes are distinguished from the two other 

 grand divisions by the undivided skull, with independent jaws, by the 

 coverin«!; of all the cartilages with a fine mosaic of tessellated particles of 

 bone, by the fixed gills with spiracles, by the presence of branchial bones, 

 by the absence of gill covers, by the extension of the labyrinth of the ear 

 out to the skin, and by the structure of the organs of generation. 



In spite of all these differences, however, there are, as already mentioned, 

 many analogies between the Selachii and the Ganoidei, these consisting of 

 the number of valves in the aorta, the muscular investment of the bulbus 

 arteriosus, which pulsates like a true heart, and of other features already 

 mentioned. The most striking difference lies in the peculiar sexual 

 apparatus of the former. The Selachians of the present day are divided 

 into two orders, the Plagiostomi and Holocephali. The former have 

 distinct jaws, and a well defined, often entirely osseous, column of vertebrae. 

 In the Holocephali, or ChimcercB, the jaws are fused to the skull, and the 

 vertebral column is only a soft vertebral cord. The two divisions are 

 represented by both fossil and recent forms in variable proportion. 



The Holocephali, in addition to the characters already mentioned, have a 

 single lateral gill opening ; two dorsal fins, the first being a simple dentated 

 spine ; the tail running out into a fine thread. The teeth are composed of 

 great plates, which rest upon the uninterrupted anteriorly prolonged base 

 of the skull ; upon the lower jaw they articulate in the cartilage of the 

 skull. The two living forms are ChimcBra and Callorhynchus, the former 

 well deserving the name. The Chimcera has a simply conical snout, the 

 second dorsal immediately behind the first, and extending to the tip of the 

 tail, which is drawn out into a long filament. ChimcBra monstrosa, the 

 only species, is abundant in the Arctic seas. Callni-hynchus has a fleshy 

 appendage to the snout ; the second dorsal commences over the ventrals, 

 and terminates opposite to the subcaudal fin. The single species, Callo- 

 rhynchus australis, is a native of the Antarctic Ocean. 



The order Plagiostomi has a cartilaginous cranium, in which the 

 individual parts are not recognisable; cartilaginous dentigerous jaws 

 attached to the cranium, also by cartilages. The face is prolonged 

 anteriorly ; and in its under side, at a greater or less distance from the 

 extremity, opens the broad transverse mouth, near to which are five or 

 more lateral spiracles or gill openings, before it the two nasal fossa. The 

 vertebral column always exhibits greater or less indication of transverse 

 separation. Ventrals and pectorals are always present, but like the other 

 fins they are soft and fleshy. The external investment consists of shagreen, 

 or of small plates variously modified. The teeth are placed on the roof of 

 the mouth and the lower jaw^ The swimming bladder is wanting, and the 

 intestine is provided with a spiral valve. We distinguish two principal 

 divisions, or sub-orders, SqualidcB, sharks, with the branchial fissures lateral, 

 eyelids free, scapular arch incomplete, pectoral fins distinct from the head, 

 body slender, fusiform ; and RaiadcB, or rays, fish with depressed body, 

 spiracles, five branchial fissures on the ventral surface of the body, beneath 

 the pectoral fins, the upper eyelid grown to the eye, or eyelids absent, 

 444 



