THE TELEOSTEI. 131 



tebrae are commonly united by zygapophyses, or oblique pro- 

 cesses, placed above the centra ; in addition to which, the 

 lower margins of the centra are, not unfrequently, united by 

 additional articular processes. Transverse processes common- 

 ly exist, but the ribs are articulated with the bodies of tlie 

 vertebne, or with the bases of the transverse processes, not 

 with their extremities. 



When a dorsal fin exists in the trunk, its rays are articu- 

 lated with, and supported by, elongated and pointed bones — 

 the interspinous bones, which are developed around preexist- 

 ing cartilages, and lie between, and are connected with, the 

 spines of the vertebrse. The fin-rays may be entire and com- 

 pletely ossified, or they may be transversely jointed and lon- 

 gitudinally subdivided at their extremities. Not unfrequentl}^, 

 the articulation between the fin-rays and the interspinous bone 

 is efi"ected by the interlocking of two rings — one belonging to 

 the base of the fin-ray and its included dermal cartilage, and 

 the other to the summit of the interspinous bone — like the 

 adjacent links of a chain. 



In all Teleostean fishes the extremity of the spinal column 

 bends up, and a far greater number of the caudal fin-rays lie 

 below than above it. These fishes are, therefore, strictly 

 speaking, heterocercal. Nevertheless, in the great majority 

 of them (as has been already mentioned, page 19), the tail 

 seems, upon a superficial view, to be symmetrical, the spinal 

 column appearing to terminate in the centre of a wedge-shaped 

 hypural bone, to the free edges of which the caudal fin-rays 

 are attached, so as to form an upper and a lower lobe, which 

 are equal, or subequal. This characteristically Teleostean 

 structure of the tail-fin has been termed homocercal — a name 

 which may be retained, though it originated in a misconcep- 

 tion of the relation of this structure to the heterocercal con- 

 dition. 



In no Teleostean fish is the bent-up termination of the 

 notochord replaced by vertebras. Sometimes, as in the Sal- 

 mon (Fig. 6, page 20), it becomes ensheathed in cartilage, and 

 persists throughout life. But, more usually, its sheath be- 

 comes calcified, and the urostyle thus formed coalesces with 

 the dorsal edge of the upper part of the wedge-shaped hypural 

 bone, formed by the anchylosis of a series of ossicles, which are 

 developed in connection with the ventral face of the sheath of 

 the notochord. 



In the caudal region of the bodj^, interspinous bones are 

 developed between the spines of the inferior arches of the ver* 



