126 A HISTORY OF FISHES 



America, has been described as operating in shoals, forcing a 

 school of Blue-fishes into a solid mass in shallow water, before 

 rushing in and seizing the prey. The Thresher or Fox Shark 

 {Alopias) feeds largely on Herrings, Pilchards, and Mackerel. 

 It swims round and round a shoal of these fishes, thrashing the 

 water with its long tail (Fig. 32A) and thus driving the pro- 

 spective victims into a compact mass, when they form an easy 

 prey. A Thresher feeding in shallow water on the coast of 

 Carolina has been described as "throwing the fish to its mouth 

 with its tail, and . . . one fish, which it failed to seize, was 

 thrown a considerable distance clear of the water." Sometimes 

 a pair of Sharks work together at this organised fishing. 



In the Nurse Sharks {Ginglymostoma) and the Hounds {Mus- 

 telus) , which include in their diet a large percentage of shell- 

 fish and crustaceans in addition to smaller fishes, there is a 

 diflferent kind of dentition. The teeth are small, pointed or 

 flattened, and adapted for grinding and crushing rather than 

 for cutting. They are arranged in pavement fashion, and all or 

 most of the rows are in use at the same time (Fig. 51B). These 

 are comparatively sluggish Sharks, feeding for the most part at 

 or near the bottom of the sea. The Port Jackson Shark {Hetero- 

 dontus), a member of the family of Bull-headed Sharks (Fig. 53B), 

 has a remarkable dentition, and provides an example of a form 

 with more than one type of tooth in the same jaw. The teeth in 

 the front of the jaws are like small cones, but farther back these 

 gradually pass into teeth which have the form of "pads" or 

 nodules of varying size (Fig. 53B). As might be supposed, this 

 curious dentition is used for grinding and crushing purposes, 

 the food consisting almost entirely of shell-fish. 



In the Guitar-fishes {Rhino batidae), Saw-fishes {Pristidae), 

 Rays {Raiidae), Sting Rays (Trygonidae) , and their allies, the 

 teeth are nearly always small, generally blunt, and arranged 

 in pavement fashion with several rows in use at once. Being 

 ground-feeders the food generally includes a high percentage 

 of molluscs, crustaceans, and other armoured creatures like 

 the sea-urchins, so that the dentition is of the crushing and 

 grinding type. In one of the Guitar-fishes, known as Rhina, 

 the tooth-covered jaws present a curious shape, the upper jaw 

 being alternately hollowed and swollen, and the lower being 

 provided with corresponding bumps and depressions to fit 

 into the upper jaw (Fig. 51c). In an allied form [Rhynchobatus) 

 the jaws are much less wavy in outline, a single swelling in the 

 lower jaw fitting into an indentation in the upper, whilst in the 



