SHARKS AND RATS. 421 



Among the more aberrant sharks is the hammer-headed 

 Sphyrna zygcena (Linn.), which grows to the length of twelve 

 feet, and is one of the most rapacious and formidable of the 

 order. 



Of the rays and skates, the saw-fish approximates most 

 to the sharks. Its snout is prolonged into a long, flat 

 bony blade, armed on each side with 

 large teeth. Pristis antiquorum 

 Latham (Fig. 390), the common saw- 

 fish, inhabits the Mediterranean Sea 

 and the Gulf of Mexico ; it is vivipa- 

 rous (Cat on.) Pristis Perroteli lives 

 in the Senegal River, while Carclmrias 

 yanyeticus is found sixty leagues from 

 the sea. 



The genuine skates or rays have the 

 body broad and flat, rhomboidal (ow- 

 ing to the great extension of the 

 thick pectoral fins). Portions of the 

 ventral fins in the males are so elon- 

 gated and modified as to form intro- 

 mittent and clasping organs. They 

 swim close to the bottom, feeding upon 

 shell-fish, crabs, etc., crushing them 

 with their powerful flattened teeth. 

 The spiracle is especially developed in 

 the rays, while, as observed by Gar- 

 man, in the majority of the sharks 

 which swim in midwater or near the 

 surfac3, the water enters the mouth 

 and passes freely out of the gill-open- 

 ings, but in the rays, which remain at 



,! Fig. 390. Beak of Saw-fish, 



the bottom, the purer sea-water enters seen from below, showing its 

 the spiracle from above to pass out of ^ 



the gill-slits. 



The smallest and most common skate of our northeast- 

 ern Atlantic coast is Raja erinacea Mitchell. It is one half 

 of a metre (twenty inches) in length, and the males are 

 smaller than the females. The largest species is the barn- 

 door skate, Raja Icevis Mitchell, which is over a metre (forty- 



