DISTRIBUTION OF FISHES IN TIME. 



429 



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 appear, and they continue to be repre- 

 sented 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 Ludlovv rocks, or at the close of the Uj per Silurian 

 epoch, there have been discovered the remains ot undoubted 

 Plagiostomous fishes, most nearly allied to the c xisting Port 

 Jackson Shark (Cestracion Philippi}. These remains consist 



-^tts^s^- 



Fig. 183. i. Spine of P leiiracanthus (one of the Rays' ; 2. Gyracanthus : 3. Ctetia- 

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

 Carboniferous Rocks. 



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 

 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 discDvery of these remains, at that 

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

 genius of Sir Roderick Murch'son, the author of "Siluria." 



Most of the fossil Elasmobranchii belong to the division 

 Cestraphori of Owen, so called because they are provided with 



