DISTRIBUTION OF FISHES IN TIME. 



379 



homocercal tails 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, most nearly allied to the existing Port 

 Jackson Shark (Cestrarion Philippt). 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 of 

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

 living shark. These have been referred to the genus Sphagodus. 

 They are the earliest known remains of Plagiostomous fishes, 

 and with the exception of the few remains from the Lower 

 Ludlow Rocks, they are the earliest known remains of fishes in 

 the stratified series. The discovery of these remains, at that 

 time the earliest known traces of Vertebrate life, is due to the 

 genius of Sir Roderick Murchison, the author of * Siluria.' 



Most of the fossil Elasmobranchii belong to the division 



Fig. 145. i. Fin-spine of Pleuracanthus (one of the Rays); 2. Gyracanthus ; 3. Ctena- 

 canthus- 4. Tooth of Petalodus ; 5. Psammodus ; 6. Ctenoptychius. All from the 

 Carboniferous Rocks. 



Cestraphori of Owen, so called because they are provided with 

 the large fin-spines, which are known to geologists as " ichthyo- 



