26 Vertebsata. 



gigantic Rhinodon of the Indian Ocean has been met 

 with sixty feet in length. The hammer-headed shark, 

 with its extraordinary bilobed head, and the saw-fish 

 (fig. 12), with its long, flattened, bony snout bearing a 

 row of strong, sharp teeth on each side, are also ex- 

 amples of the order. 



The skates, or rays, are remarkable for their flat- 

 tened form, due largely to the enormous size of the 

 pectoral fins. One form, rarely found in British seas, 

 though common in the Mediterranean, is the torpedo 

 or electric ray, which has near its head two large 

 electric batteries, whereby the fish can give severe 

 electric shocks; these organs are joined to the brain 

 by large nerves, and consist of closely apposed 

 columnar elements, which morphologically are con- 

 sidered to consist of extremely modified muscular 

 tissue. 



CHAPTER V. 



ORDER 3, GANOID FISHES ; ORDER 4, BONY FISHES ; 

 ORDER 5, DIPNOI. 



16. Order 3, Ganoidei. The living fishes of this 

 group are very few (about thirty), and most of them 

 are inhabitants of rivers or of lakes but in former 

 times they were apparently numerous and rich in 

 species, as the fossil remains of six hundred species 

 are already known. They are characterised by pos- 

 sessing ganoid scales that is, brightly polished plates 



