FISHES. 



313 



Fig. 278. Tails of different fishes. 

 a Homocercal tail (Sword-fish) ; b Het- 

 erocercal tail (Sturgeon.) 



the tail are equal, and the vertebral column, instead of being 

 prolonged into the upper lobe of the tail, stops short at its 

 base. In the heterocercal tail, on the other hand, the vertebral 

 column is prolonged into the up- 

 per lobe of the tail, so that the 

 tail becomes unequally lobed, its 

 greater portion being placed be- 

 low the spine. Even where the 

 vertebral column is not pro- 

 longed into the upper lobe, the 

 tail may nevertheless become 

 heterocercal, in consequence of 

 a great development of the 

 haemal spines as compared with 

 the neural spines of the vertebrae. 

 As regards their general dis- 

 tribution in time, the geological 

 history of fishes presents some 

 points of peculiar interest. Of 

 all the classes of the great sub- 

 kingdom Vertebrata, the fishes 

 are the lowest in point of or- 

 ganisation. It might therefore 

 have been reasonably expected that they would present us 

 with the first indications of vertebrate life upon the globe ; 

 and such is indeed the case. After passing through the enor- 

 mous group of deposits known as the Laurentian, Huronian, 

 Cambrian, and Lower Silurian formations representing an 

 immense lapse of time during which, so far as we yet know, 

 no vertebrate animal had been created we find in the Upper 

 Silurian Rocks the first traces offish. The earliest of these, in 

 Britain, is found in the base of the Ludlow Rocks (Lower Lud- 

 low Shale), and belongs to the placoganoid genus Pteraspis. 

 Also in the Ludlow Rocks, but at the summit of their upper 

 division, are found fin-spines and shagreen, probably belonging 

 to Cestraciont fishes that is to say, to fishes of as high a 

 grade of organisation as the Elasmobranchii. So abundant 

 are the remains of fishes in the next great geological epoch 

 namely, the Devonian or Old Red Sandstone that this period 

 has frequently been designated the " Age of Fishes." Most of 

 the fishes of the Old Red Sandstone belong to the order Gan- 

 oidei. In the Carboniferous and Permian Rocks, which close 

 the Palaeozoic period, most of the fishes are still Ganoid, but 

 the former contain the remains of many Plagiostomous fishes. 

 At the close of the Palaeozoic and the commencement of the 



