VERTEBRATA: FISHES. 505 



existence in past time. Judging from analogy, however, it is 

 highly probable that both of these must have had a vast 

 antiquity, and it is not impossible that the so-called " Cono- 

 donts " of the Palaeozoic period may yet be shown to be the 

 teeth of fishes allied to the Lampreys. 



Leaving these unrepresented orders out of consideration, the 

 following are the chief facts as to the geological distribution of 

 the other great groups : 



I. Ganoidei. As far as is yet known with certainty, the 

 oldest representatives of the fishes belong to this order. The 

 order is represented, namely, in the Upper Silurian rocks 

 by the remains of at least four genera. In the Devonian 

 rocks, or Old Red Sandstone, the Ganoids attain their maxi- 

 mum both in point of numbers and development. The 

 Placoganoid division of the order is represented by the 

 singular genera Pterichthys, Cephalaspis (fig. 267), Pteraspis, 

 and Coccosteus. The Lepidoganoid division of the order is 

 now also abundantly represented for the first time, the genera 

 Dipterus, Osteolepis (fig. 265), Glyptolepis, Holoptychius, Dipla- 

 canthus, and many others, belonging to this section. As 

 regards the further distribution of the Placoganoids, the section 

 of the Ostracostci, characterised by the great development of 

 the cephalic buckler, appears to have died out at the close of 

 the Devonian period. The other section, however namely, 

 that of the Sturionidce is represented in the Liassic period 

 (Mcsozoif) by the genus Chondrosteus, and in the Eocene 

 (Kainozoic] by a true Sturgeon, the Acipenser toliapicus. 



The Lepidoganoids continue from the period of the Old Red 

 in great profusion, and they are represented by very many 

 genera in the Carboniferous and Permian rocks. In the earlier 

 portion of the Mesozoic period /.<?., in the Lias and Trias 

 they are still represented, but all the forms are as yet hetero- 

 cercal. In the Oolitic rocks, for the first time, Lepidoganoids 

 with homocercal tails (fig. 266, B) appear, and they continue 

 to be represented up to the present day. 



II. Elasmobranchii. Like the Ganoidei, the great order of 

 the Sharks and Rays is one of vast antiquity. At the top of 

 the Upper Ludlow rocks, or at the close of the Upper Silurian 

 epoch, there have been discovered the remains of undoubted 

 Plagiostomous fishes, mostly nearly allied to the existing Port 

 Jackson Shark (Cestracion Philippi). These remains consist 

 chiefly of defensive spines, which formed the first rays in the 

 dorsal fins, and upon these the genus Onchus has been founded. 

 Besides these there have been found portions of skin or 

 " shagreen," with little placoid tubercles, like the skin of a 



