The True Sharks 



553 



limp. For these Garman has proposed the generic name Mala- 

 corhinus, a name which may come into general use when the 

 species are better known. In the deep seas rays are found 

 even under the equator. In the south-temperate zone the 

 species are mostly generically distinct, Psammobatis being a 

 typical form, differing from Raja. Discobatus sinensis, com- 

 mon in China and Japan, is a shagreen-covered form, looking 

 like a Rhinobatus. It is, how r ever, a true ray, laying its eggs 

 in egg-cases, and with the pectorals extending on the snout. 

 Fossil RajidcE, known by the teeth and bony tubercles, are 

 found from the Cretaceous onward. They belong to Raja and 

 to the extinct genera Dynatobatis , Oncobatis, and Acanthobatis . 

 The genus Arthropterus (rileyi), from the Lias, known from a 

 large pectoral fin, with distinct cylindrical- jointed rays, may 

 have been one of the Rajida, or perhaps the type of a distinct 

 family, Arthropteridas. 



Narcobatidae, or Torpedoes. The torpedoes, or electric rays 

 (Narcobatidce), are characterized by the soft, perfectly smooth 



Fio. 345. Numbfish, Narcine brasiliensis Henle, showing electric cells. 

 Pensacola, Fla. 



skin, by the stout tail with rayed fins, and by the ovo viviparous 

 habit, the eggs being hatched internally. In all the species is 

 developed an elaborate electric organ, muscular in its origin 

 and composed of many hexagonal cells, each filled with soft 

 fluid. These cells are arranged under the skin about the back 



