78 



SKATES. 



The distitiguishing marks of this family as compared with 

 the Sharks, are a general depression and flatness of the body, 

 with a large expansion of the pectoral and ventral fins; the 

 former being united to and encompassing the head, so that 

 together they form a disc, which is either rounded or of a 

 rhomb-like form, with tlie angles slightly marked; a structure 

 that displays a remarkable adaptation to the peculiar habits of 

 this tribe. The snout in most of the species projects, and in 

 all the mouth and nostrils are under the head, the latter 

 being united to the former by a superficial channel, or deep 

 fold in the skin. The gill openings are five in number, as 

 in the Sharks, but they are placed on the under surface; and 

 on the head, not far behind the eyes, are the spiracles or 

 temporal orifices, which, when they exist in the Sharks, are 

 generally small, but in this family they are large and open, 

 although capable of being closed at the will of the creature; 

 and their use is more apparent in the present class; for as it 

 is their habit to lie flat on the ground, it becomes necessary 

 that they should be furnished with an apparatus sufficient to 

 give passage to a full supply of water to the gills for the 

 purpose of breathing; at a time when the mouth must for a 

 long time remain shut, and the current from that direction 

 consequently be interrupted, even if it ever exist. That this 

 supply of water needs to be large and constantly repeated, 

 may be judged from what Monro describes of the extensive 

 capacity of the interior structure of the gills, the superficial 

 extent of which he estimates as amounting to almost the whole 

 external surface of the human body. Thus, says he, in each 

 side of the body of a Skate there are four double gills, or 

 gills with two sides each, and one single gill, on which the 

 branchial artery (which brings the blood for purification) is 

 spread out. On each of these sides there are about fifty 



