SHARKS. 101 



the general form of their body. In the Sharks, or Selachoidei, the 

 body is elongate, more or less cylindrical, gradually passing into 

 the tail; their gill-openings are lateral. In the Rays, or Batoidei, 

 the gill-openings are always placed on the abdominal aspect of the 

 fish ; the body is depressed, and the trunk, which is surrounded 

 by the immensely developed pectoral fins, forms a broad flat disk, 

 the tail appearing as a thin and slender appendage. However, 

 some of the Rays approach the Sharks in having the caudal 

 portion less abruptly contracted behind the trunk. Fossil Plagio- 

 stomes are very numerous in all formations, but in the oldest the 

 only remains they have left consist of teeth and fin-spines. Some 

 of the earliest determinable fish-remains are believed to be, or are, 

 derived from the Plagiostomes. The recent forms, of which nearly 

 300 species are known to exist, are arranged in the following 

 families : — 



A. Selachoidei, or Sharks. 

 1. Carchariidcs. 2. Lamnidce. S.[Rhinodontidce. 4. Notidanidce. 

 5. Scylliida?. 6. Cestraciontidce. 7. Spinacidce. 8. Rhinida. 

 9. Pristiophorida. 



B. Batoidei, or Rays. 

 10. Pristida?. 11. Rhinobatida. 12. Torpedinidae. 13. Raiid<s. 

 14. Trygonidce. 15. Myliobatida. 



** 



A. Selachoidei : Sharks. 



Sharks are most numerous in the tropics, and become scarcer 

 beyond ; they are exclusively carnivorous, and those armed with 

 powerful cutting-teeth are the most formidable tyrants of the 

 ocean. Many of the smaller kinds are eaten. Sharks' fins form 

 in India and China an important article of trade (specimens of 

 this article are exhibited in Case 32). Sharks have no scales, like 

 other fishes; their skin is covered with calcified papillae, and if 

 these papillae are small, pointed, and close-set, the skin is called 

 " shagreen." 



Carchariidce (Cases 31-34). — To this family belong the true [Case 

 Sharks (Carcharias) , common in the tropical, but less so in the 

 temperate seas. Numerous species are distinguished, of which 

 one of the most common is the Blue Shark (C. glaucus). Indi- 



