342 PECULIARITIES OF PLAICE FPvOM DIFFERENT FISHING GROUNDS. 



cycloid scales, and to have come from the far north ; to have formed the 

 variety glacialis on the Arctic coasts, and then with this variety to have 

 entered the ]>altic, where the plaice gave rise to the Baltic form of 

 plaice, and the glacialis to the flounder. 



The conclusions at present suggested by the facts to my own mind 

 are as follows : The plaice is by no means necessarily the original form, 

 as there is reason for holding that the original form of these and other 

 flat-fishes had ctenoid scales of the usual kind, as in the dab. The facts 

 show that the species glacialis is the Arctic form, the flounder the 

 fluviatile form of the plaice. Whatever the causes which led to the 

 reduction of the scales in the plaice, it is certain that the ctenoid 

 condition is more developed in the Arctic form : this form is also 

 without tubercles and has fewer fin-rays. There are objections to 

 the view, which Duncker appears to take, that the development of 

 the tubercles in the flounder is a further stage in the development 

 of the spinules on the scales in the plaice and in glacialis. The most 

 spiuulated scales occur in the two latter forms on the fin-rays and 

 in the interspinous regions, while in the flounder it is precisely in these 

 two regions that the scales are most rudimentary, and along the base 

 of the fins and along the lateral line the scales are developed into spiny 

 tubercles. As we have seen, the spinules on the fin-rays are retained to 

 some extent only in the smoothest flounders, those of the Mediterranean, 

 while in those of more northern latitudes the fin-rays are scaleless. 

 Thus we might almost say that the condition of the flounder was due to 

 the further progress of a modification in the same direction as that 

 of the plaice, that the ctenoid scales first underwent reduction, and then 

 when they had become rudimentary some of them in particular parts of 

 the body developed into tubercles. This view, however, is not consistent 

 with the fact that both plaice and flounder become rougher, their 

 calcified skin armature more developed, in the north than in the south. 

 The correct interpretation of this fact is evidently that the development 

 of scales has taken a different direction in the flounder, and that in both 

 directions cold, or some condition accompanying a northern climate, 

 has the effect of producing enlargement of the structures connected with 

 the scales. It is not possible at present to see any connection between 

 the fact that the flounder lives in rivers, and the peculiar development 

 of its tubercles, nor can we see any advantage to the fish in the 

 possession of these structures. 



We do not find that the correspondence which is observed between 

 climate and development of spines on the scales in the plaice and 

 flounder exists when we compare the species of flat-fishes with one 

 another. The sole, for instance, is a distinctly southern form, and its 

 scales are strongly ctenoid all over the body. The dab, however, is 



