150 THE ANATOMY OF VEETEBEATED ANIMALS. 



existing cartilages, and lie between, and are connected with, 

 the spines of the vevtebrai. The fin-rays may be entire 

 and completely ossified, or they may be transversely jointed 

 and longitudinally subdivided at their extremities. Not 

 unfrequently, 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, p. 16), 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 characteris- 

 tically Teleostean structure of the tail-fin has been termed 

 homocercal — a name which may be retained, though it 

 originated in a misconception of the relation of this struc- 

 ture to the heterocercal condition. 



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

 notochord replaced by vertebras. Sometimes, as in the 

 Salmon (Pig. 6, p. 17), it becomes ensheathed in cartilage, 

 and persists throughout life. But, more usually, its sheath 

 becomes calcified, and the urostyle thus formed coalesces 

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

 hypui-al bone, formed by the ankylosis 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 body, interspinous bones are 

 developed between the spines of the inferior arches of the 

 vertebrae, and bear the fin-rays of the anal and, in part, of 

 the caudal fin. 



The Teleostei differ very much in the extent to which the 

 primordial cranium persists throughout life. Sometimes, 



