VERTEBRAL COLUMN 53 



with in certain Palaeozoic Fishes. In most other Fishes the 

 ventral part of the tail-fin with its supporting skeleton, as a 

 result of unequal growth, is more strongly developed than the dorsal 

 part, and the vertebral column becomes bent up dorsally, giving 

 rise to a Iwterocercal tail. This form of tail may be recognised 

 externally in most Elasmobranchs, Ganoids, and numerous fossil 

 Fishes, or may be masked by a more or less symmetrical tail-fin, 

 as in Lepidosteus, Amia, and more particularly in most Teleosts, 



FIG. 43A. TAIL OF Lepidosteus. 



h 



n 



FIG. 43B. CAUDAL END OF VERTEBRAL COLUMN OF SALMON. (From Boas's 



Zoology.) 



h, centrum ; h', urostyle ; ?i, haemal arch ; ri, hypural bone ; o", neural arch ; 



t, neural spine. 



in which the heterocercal character is only visible internally, 

 and the tail is described as homocercal (cf. Figs. 43, A and B). 

 The posterior end of the vertebral column is then frequently 

 represented by a rod-like urostyle, and in Teleosts one or more 

 wedge-shaped hypural bones (enlarged haemal arches) generally 

 occur directly beneath it. 1 



1 The diphycercal character of the tail in Dipnoi and certain Teleostomi is 

 probably not primitive (protocercal), but has been acquired secondarily. 



