ELECTRIC RAYS. 45 



advances. The condition found in Aetobatis (122), where there 

 are no marginal teeth, thus marks but a further step in the same 

 direction of specialisation. In the Devil-fish, on the other hand, 



Fig. 23. — Jaws of Eagle-Ray, Myliobatis aquila, seen from behind. 

 (From Giinther, " Study of Fishes.") 



the teeth are numerous and small. In the extinct (Cretaceous) 

 genus Ptychodus the teeth are quadrate in form, with an elevated 

 crown sharply separated from the root (124 a and b). The crown 

 has a series of transverse or radiating ridges, surrounded by a more 

 finely corrugated marginal area. 



In the Torpedinidae. or Electric Rays (fig. 24, p. 46) the disc is Electric- 

 broad, and the skin smooth and soft. The tail has a longitudinal > ay * 

 fold on each side ; the caudal fin is present and usually two dorsal 

 fins also. The skeleton of the pectoral fin is not continued 

 forwards beyond the base of the snout. An electric organ capable 

 of giving an electric shock is present between the head and the 

 pectoral fin of each side. The organ (see 1.20) consists of closely- 

 set hexagonal prisms, vertically disposed, and terminating against 

 the skin of the upper and under surfaces of the body. The electric 

 organ is supplied by branches of the fifth and tenth cranial nerves 

 (trigeminus and vagus). The Torpedo occurs in the Mediterranean 

 Sea and the Indian and Atlantic Oceans (e. g. Torpedo marmorata, 

 119, and fig. 24) ; one species {Torpedo hebetans) is occasionally 

 found off the coast of England (see specimen 1080 on the floor 

 beneath the tail of the skeleton of the Basking Shark, within the 



