SKIN, SCALES, AND SPINES 



87 



Other species of the same genus each principal spine has smaller 

 accessory spines developed round its base. In the Torpedoes 

 {Torpedinidae) , on the other hand, and in some of the Sting 

 Rays (Trygonidae) and Eagle Rays (Myliobatidae) , the denticles 

 have been discarded and the skin is quite smooth. 



Mention may be made here of the tail-spine or "sting" of the 

 Sting Rays and related forms, which in these fishes takes the 

 place of the dorsal fins. Its origin is somewhat obscure, but it 

 may have arisen through the enlargement or fusion of certain 

 denticles in the tail region. It is generally serrated along both 

 margins, and may be as much as from eight to fifteen inches in 

 length. It provides a formidable weapon, and when the tail is 

 lashed from side to side or curled round the intended prey, 



Fig. 38. 

 Saw-fish {Pristis pectinatus), X ijV- 



inflicts painful jagged wounds. The "stings" are shed from 

 time to time, and replaced by new ones growmg from under- 

 neath; sometimes two or three may be present in one fish at 

 the same time (Fig. 36e). . 



In the Saw-fishes [Pristis), ray-Hke fishes found m all warm 

 seas, the snout is produced to form a long flat blade, armed on 

 either margin with a series of strong tooth-like structures (Fig. 

 38). These are modified dermal denticles, and are not only 

 very much enlarged, but are firmly implanted in sockets m the 

 cartilage of the rostrum. Saw-fishes grow to a large size, 

 specimens twenty feet in length being quite common and 

 "saws" six feet in length and a foot across the base are by no 

 means rare objects in the windows of curio or natural history 

 dealers. Normally, this effective weapon is brought into use 



