RAIN^^ OR TYPICAL RAYS. 



173 



upon the tail, therefore, appears to us a more certain 

 mark for discriminating the two typical genera of Try- 

 gon and Pastinaca, than the number of the stings ; and 

 this view, we perceive, has been taken by MuHer and 

 Henle. In furtlier proof, also, we may refer to the 

 two species above mentioned, from India : both are of 

 the same form, and both have the tail entirely naked ; 

 yet in one there is but a single spine, while the other 

 has two. The third genus is that of Pteroplatea, — a 

 name given by the last mentioned ichthyologists to 

 certain sting rays, which have the pectorals so very long 

 as to render the breadth of the fish considerably more 

 than its length : the tail, hke that of Pastinaca, is 

 always naked, but it is also remarkably short. It is here 

 that the stings begin to disappear; for although one 

 species of those which have been described possesses two, 

 yet in another, from India (RusseU, pi. 22.), there is 

 none whatever. This latter fish, therefore, brings us near 

 to the genus Raia {R.rubra,Ag. 18.), as now restricted 

 and understood by the moderns. This group, indeed, has 

 recently been divided into several sub-genera ; but as 

 the value of these remains to be determined by a philo- 

 sophic analysis of the 

 real types, we do not, 

 at present, adopt them. 

 The whole may be 

 characterised as dia- 

 mond-shaped fishes, 

 almost always covered 

 with prickles or mi- 

 nute asperities, but 

 never having the tail 

 armed with a barbed 

 spine, as in the three 

 preceding genera : the 

 tail, moreover, terra i- 

 . nates in a small cau- 

 dal fin ; immediately 



