SHARKS AND RAYS. 231 



CHAPTER XII. 



Sharks and Rays Lampreys and Hag-Fishes The Lampreys 

 The Lancelets. 



SHARKS AND RAYS. 



( Sub-class : HLASMOBRANCHII. ) 



IN both their living and extinct forms, the Sharks and Rays 

 are remarkable, in that they do not possess a bony skeleton, 

 such as that possessed by all the fishes with which we have 

 so far been dealing. Instead, we find that it is wholly car- 

 tilaginous or partly calcified (that is, hardened by the addi- 

 tion of calcareous or limey matter), while the skin takes 

 the form of a more or less uniform coating of dermal den- 

 ticles, which under the microscope show a structure similar 

 to that of teeth. If these denticles are small, pointed and 

 close-set, the skin is called "shagreen." In some cases large 

 tubercles, plates or spines of various sizes, are found to be 

 developed. 



The Elasmobranchs as the Sharks and Rays are 

 called are chiefly active predaceous fishes, occurring in 

 nearly every sea of the world, and also in many fresh- 

 waters ; even in some cases living entirely in the latter. As 

 compared with other fishes, none of them are very small; 

 while some of them are exceedingly large, and amongst the 

 Sharks are included the largest of all living fishes. In 

 regard to their bathymetrical distribution, it may be men- 

 tioned, that they are at present known to occur in depths 

 ranging from the shallowest water, down to nearly 1000 

 fathoms. Though they are most abundant within tropical 

 or semi-tropical areas, they are quite numerous both as 

 regards species and individuals in temperate regions, while 

 certain kinds are known to inhabit permanently, waters lying 

 within the Arctic and Antarctic Circles. 



So far as is at present known, all Elasmobranchs are 



