4.04 



MANUAL OF ZOOLOGY. 



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 {Cestracion Philippi). These remains consist 

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

 dorsal fins, and upon these the genus Onchus'hzs, 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 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 

 Cestraphori of Owen, so called because they are provided with 

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



Fig- 153- — !• Fin-spine of /'/ff«racrt«/^wj (one of the rays); 2. Gyracnnihus ; 3. Ctena' 

 cantkus; 4, Tooi^oi Petatodus; 5. Psammodus; 6. Ctenoptychius. All from the 

 Carboniferous Rocks. 



doruliles.'' 'The two families of this division — the Cestracionts 

 and Hybodonts — are largely represented in past time, the 

 former chiefly in the Palaeozoic period, the latter chiefly in the 



