THE FISHES 



I6S 



is commonly covered with placoid scales which are pecu- 

 liar in having a sort of prominence, or denticle situated 

 upon a flattened base. Various transitions between these 

 scales and teeth can be traced in some forms, so that we 

 may regard teeth and scales as corresponding or homolo- 

 gous organs. 



The elasmobranchs with a single exception are all marine 

 and carnivorous. The sharks are, as a rule, active, preda- 



FIG. 133. A skate. 



tory animals feeding mostly upon other fishes. The whale 

 shark Rhinodon may reach a length of 40-50 feet and the 

 large, white, man-eating shark, Carcharias, 25 feet. 

 In the skates and rays the body is remarkably flattened, 

 and adapted to living on the bottom where the animals 

 feed mainly upon shell-fish and crustaceans. In the sting- 

 rays there is a pointed spine near the base of the tail, which 



