56 THE STORY OF FISH LIFE. 



slight upward bend of the vertebral column, with 

 a large ventral and a smaller dorsal lobe. Such a 

 tail is seen in the ancient sturgeon-like forms, and 

 the sharks, and persists to this day in their living 

 representatives. The homocercal type was, how- 

 ever, well on the way towards perfection so far 

 back as that period of the world's history known 

 as the Lower Lias. Many of the fishes of the 

 ancient seas of that time, such as the Da/pedius, 

 have acquired an almost perfect homocercal 

 tail. But we may confidently believe that the 

 homocercal tail of the ancient Dapedius acquired 

 its special characteristics in precisely the same 

 way as the flounder — by a gradual passage from 

 the primitive diphycercal to the ultimate homo- 

 cercal type, through heterocercy. 



But one or two of the ancient fishes who 

 swam about in the ancient seas, whose dried and 

 hardened floors now form the rocks of what we 

 call the Triassic and Liassic formations, were blessed 

 with two tails apiece, or, to be quite correct, with 

 two tail-fins. The most interesting of these was 

 that of the Diplurus. In this fish the true tail 

 slowly dwindles in size, terminating in little more 

 than a filament, bearing a tiny tail-fin. But in 

 front of this we find what was probably the 

 functional tail-fin, and this fin appears to have 

 derived its origin in a rather curious way. It 

 will be remembered that the fins which project 

 from the middle of the back are known as the 

 dorsal fins, and may vary in number; whilst 

 the fins which project from the middle of the 

 abdomen are known as the anal fins. Now, in 

 Diplurus, the hindmost dorsal and anal fins in- 



