SHARKS AND SKATES. 



73 



occasion offers, do not hesitate to attack man. Many are the cases in history and in 

 fiction where sharks have eaten a human being in one or two mouthfuls. 



In sliape the sharks are long and cylindrical, the body terminating in a sharp snout 

 in front and in a long and flattened tail behind. The mouth is transverse, and usually 

 placed on the lower surface of the head some distance behind the tip of tlie snout ; 

 the gill-elefts vary between five and seven in number and always occupy a lateral 

 position ; the edges of the eyelids are free ; and the shoulder girdle is incomplete. The 

 differences between the typical forms of sharks and skates are very evident, but a 

 series of intermediate forms intergrade between the Squall and the Raia;, so that 

 almost the only certain external ch.iracler sei)arating the two is found in the position 

 of the gill slits ; lateral m the sharks, ventral in the rays. 



Aimost all the sharks are marine, though many occasionally follow their prey into 

 the mouths of rivers, where the water is brackish, or even fresh, and one species 

 {Eulamia nicaratjuensis) occurs in Lake Nicaragua, where it is cut off from the ocean 

 by a river a hundred miles in length. Many species are pelagic, that is, they live in 

 the open seas, following schools of fish or ships for weeks, in the latter case feeding 

 upon the refuse from the cook's galley or on anything else that may fall overboard. 

 Other species are found near the shore, living near the bottom, and only rising to the 

 surface in the pursuit of food. The smaller species often form immense schools, which 

 follow the mackerel or other migratory fishes. The majority of species of sharks are 

 found between the tropics. In the temperate regions they are not so numerous, 

 while the sleeper shark (Sonuiiosus microcejy/udas), the black dog-fish {Centrosci/llmm 

 fubricii) and the basking shark 

 ( Ceturhinus maxiniKs), enter 

 the Arctic seas. Few of them 

 descend into deep water, only 

 Centroscynmus coelolepis being 

 known from a depth of five 

 hundred fathoms. 



The teeth of sharks are es- 

 pecially interesting, from the 

 method of replacement of those 

 worn out or torn out. In the 

 majority of sharks tlie teeth 

 are more or less triangular in 

 outline, and are movably artic- 

 ulated with the jaws. They 

 are arranged in several rows, 

 one behind another, the first 

 or outer row alone being used, 

 while the rest are turned back 

 out of the way. When those 

 of the first row are lost, those 

 of the next row rise up to take 

 their j)lace. In some shai-ks, however, the teeth develop a pavement much like that 

 characteristic of the ra3's. 



Sharks have but a limited economic value. In the eastern countries many of the 

 smaller species are eaten, while the fins, with their abundant cartilage, form the basis of 



Fig. 58. —Jaws .ii 



single teeth are of the natural size. 



dtarhius glauotis; the 



