WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 5 



character. These also overlap, the dorsals varying in both forms from 

 68 to 77 and the anals from 50 to 61, the dab usually presenting the 

 higher number. The flounder has from 58 to 64 dorsal rays and 

 from 38 to 46 anal rays. 



Pseudopleuronectes is intermediate in the number of fin rays between 

 P. flesus and P. platessa. It also turns at an intermediate length. 

 Taking Petersen's figures for Denmark, P. flesus turns at 8 mm. and P. 

 platessa at from 10 to 11 mm. The length at which my shorter larvae 

 turned was from 8 to 9 mm. No individuals longer than this were 

 found metamorphosing until the length of about 14 mm. was reached. 



Limanda ferruginea has more fin-rays than P. limanda. If I am cor- 

 rect in the assumption that the larger, more bulky fish, which turns at 

 a length of 14 to 15 mm., is the young of Limanda, its length at meta- 

 morphosis would be intermediate between those found for P. limanda 

 by Kyle and by Petersen. 



If this fish is the young of Limanda, another problem would be 

 solved. How is it that, with two such distinct sizes at metamorphosis, 

 the small flatfishes seined a month later are about uniform in size 1 

 Limanda is a comparatively deep-water fish, being found in the deepest 

 parts only of Vineyard Sound ; the young may have returned by the last 

 of July to the region where the adults live, so that there would be left ■ 

 only the young of the on-shore species, P. americanus. 



That I took only a few specimens of these problematical coarser larvae 

 in June, 1898, and that half the larvae taken in the same month of the 

 next year were of this kind, leads me to believe that the breeding sea- 

 sons of P. americanus and Limanda may not always exactly coincide. 

 This question can very easily be settled by breeding the fish, and satis- 

 factorily only in that way. It may be that the phenomena we have to 

 deal with here are explainable in another way. Looss ('89) found that 

 tadpoles metamorphosed in " waves," a part only of a brood changing 

 at a time. There might be something of this sort here, metamorphosis 

 at the one length or at the other depending on the advancement of 

 development. 



I wish to thank Mr. Alexander Agassiz for the privilege of occupying 

 one of the Museum tables at the U. S. F. C. laboratory during parts of 

 the summers of 1898 and 1899, and Mr. W. A. Willard for a number of 

 brains of adult fishes. The work on the nervous anatomy was done, in 

 part, under the direction of Dr. G. H. Parker. I am deeply indebted 

 to Dr. E. L. Mark, at whose suggestion the work was undertaken, for 

 useful advice and the supervision of the whole work. 



