l8 SORTATION. 



"inferior," that is, on the under-side of the body, and they .ire 

 divisible into two well-marked groups 



1. Those in which the tail has no longitudinal fold. 



2. Those in which the tail has a longitudinal fold. 



In both groups the back fin or " dorsal " is either placed on 

 the tail or, as in one genus, is absent altogether. The genus 

 having no dorsal belongs to the first group. It is Trygon, arid 

 the only species found in British waters, and that not a frequent 

 one, is T. pastinaca, the sting-ray, which has the long spine sticking 

 out of the tail that has often done duty as a spear-head in the 

 hands of Telegonus and others, it being a weapon to beware 

 of, and much more curious to look at than pleasant to make 

 acquaintance with. 



In the same small group are two genera, both comprising what 

 are known as devil-fishes, in which the tail carries a dorsal fin. 

 In one, Cephaloptera, that fin is between the ventrals ; in the other, 

 Myliobatis, it is beyond the ventrals. Of each genus there is but 

 one species on our list, and they are strikingly different in 

 appearance, the first, C. giorncz, the ox-ray, having a stumpy tail 

 behind and two horn-like projections on the head; the other, 

 M. aquila, the whip-ray or eagle-ray, having a long whip-like tail, 

 a projecting snout, and no horns. Both species attain large 

 dimensions, but not in British waters; the whip-ray has been 

 caught fifteen feet long, and the ox-ray runs larger, but not so 

 large as another North Atlantic species, C. diabolus, which, off the 

 coast of Florida, has swallowed an anchor as if it were a fish- 

 hook, and swum out to sea with the luckless craft that only 

 recovered its independence when its captor died. 



As we have talked about dorsals and slipped by ventrals without 

 explanation, we had better pause for a moment for a few words 

 on the fin question. The typical fish has two sets of fins, the 

 unpaired or " vertical " or " median " fins, and the paired fins, 

 which are the homologues of the limbs of the higher vertebrates, 

 the " pectorals " answering to the arms and the " ventrals " to the 

 legs. The ventrals are not happily named ; they shift their position 

 considerably, and, as we shall subsequently find, their position is 

 an important aid in identification. It is becoming customary to 

 call them " pelvics," but, unfortunately, there is what is called 

 a "fin formula" in icthyology D. P. V. A. C., in which D stands 

 for dorsal, P for pectorals, V for ventrals, A for anal, and C for 

 caudal, a formula which would be all the better were it to read 

 D. A. C. P. V., as we have made it do further on, in which the 

 vertical or body-fins come first, and the paired or limb-fins last. 

 If we use pelvic instead of ventral we shall have two P's, and a 

 source of confusion when comparing with previous books that has 

 led us to retain the old style though preferring the new. 



In the earlier, as in many of the existing fishes, the vertical 

 fins are all in one. In time the one continuous fin became divided 

 up into " dorsal," the back fin ; " caudal," the tail fin; and " anal," 

 the fin between the vent and the tail ; and in some cases the 

 dorsal became divided into two or even three portions. Examples 



