1 82 EVOLUTION OF ELASMOBRANCHS vi. 5-6 



a plankton feeder and becomes very large. It is not closely related to 

 the basking sharks. It moves up and down vertically, the mouth open, 

 sucking in plankton. In this group there are also many of the fiercest 

 man-eating sharks, such as Carcharodon, often 30 ft long, found in 

 many seas. Some fossil forms of this genus are estimated to have 

 reached a much greater length, possibly of 90 ft. 



The division Squaloidea includes those sharks in which there is 

 a spine in front of each dorsal fin. They are not, however, otherwise 

 different in habits from the other sharks. The spiny dogfish (Squalus) 

 is a well-known type and here belong also the saw-sharks (Pristio- 

 phorus) and a group of bottom-living forms, the angel-fishes or monks 

 [Squatina), which acquire a superficial similarity to the skates and 

 rays. Alopias, the thresher, is said to differ from most sharks in that 

 instead of seizing the prey as it is presented, it hunts systematically, 

 several sharks working together and using their whip-like tails to 

 drive smaller fishes such as mackerel into shoals, where they are then 

 seized. 



6. Skates and rays 



The Hypotremata, skates and rays, have become specialized for 

 life on the bottom of the ocean in shallow waters, feeding mainly on 

 invertebrates, and usually having blunt teeth (Fig. 114). Locomotion 

 is no longer by transverse movements of the body but by waves that 

 pass backwards along the fins. In the earlier stages, such as Rhino- 

 bath, the banjo-ray, which has existed from the Jurassic period to the 

 present, the edges of the fins are still free and the tail is well developed. 

 In Pristis, another saw-fish type, outwardly similar to Pristiophorus 

 and known since the Cretaceous, the head is drawn out into a long 

 rostrum armed with denticles. Its use is uncertain but the head strikes 

 from side to side among shoals of fishes. There are species in India, 

 China, and the Gulf of Mexico that live in fresh water. In Raja, first 

 found in the Cretaceous, the pectoral fins are attached to the sides of 

 the body and the median fins are very small, whereas in the more 

 recent Trygon and other sting-rays the tail is reduced to a defensive 

 lash, the dorsal fin persisting as a poison spine. In the eagle-rays 

 (Myliobatis) the teeth are flattened to form a mill able to grind mollusc 

 shells (Fig. 114). The sea-devils (Mobula) have expansions of the fins 

 at the front of the head, which they use to chase fishes to the mouth, 

 hunting in packs. In Torpedo, the electric ray, the fins extend so far 

 forward that the front of the animal presents a rounded outline. The 

 animal is protected by a powerful electric organ, formed by modified 



