240 ZOOLOGY. 
The genuine cartilaginous fishes are distinguished from the two other 
grand divisions by the undivided skull, with independent jaws, -by the 
covering 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 Ganotdei, 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 vertebre. 
In the Holocephali, or Chimera, 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 Chimera and Callorhynchus, the former 
well deserving the name. The Chimera 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. Chimera monstrosa, the 
only species, is abundant in the Arctic seas. Callorhynchus 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 smal] 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, Squalida, sharks, with the branchial fissures lateral, 
eyelids free, scapular arch incomplete, pectoral fins distinct from the head, 
body slender, fusiform; and Kaiade, 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, 
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