218 Proceedings of the Royal Physical Society. 



to more than in the embryo described, the two sides are 

 equally closed."* 



In remarking upon this paper, I may commence by say- 

 ing, that here, for the first time, do we find distinctly an- 

 nounced the fact and doctrine, that the dorsal fin is not 

 primarily advanced so far forwards in the head as we find 

 it in the fully developed flat-fish, but that it advances after 

 the eyes have turned round, and that it then advances 

 straight forwards, regardless of the deviation of the original 

 middle line of the head. Thus we are afforded a ready and 

 rational explanation of the difficulty which met us at first, 

 namely, as to how, if the middle line of the top of the head 

 has been twisted to one side, the dorsal fin, a mesial struc- 

 ture, has not followed that twisting. Yan.Beneden, however, 

 is not the first to notice an occasional condition of the adult 

 flat-fish, similar to that which he has described in his embryo, 

 but it seems to me to be the first to appreciate properly the 

 morphological value of such phenomena. 



Schleep, in " Oken's Isis," for 1829. p. 1049, has described 

 and figured an adult turbot, similar to the one referred to 

 by Yan Beneden, and to which he has given the name of 

 Pleuronectes maximus duplex. The two eyes are still on 

 each side of the head, the upper is just about to make 

 the turn, and the anterior part of the dorsal fin has not yet 

 advanced beyond the eyes, but projects over them in a free 

 pointed process. Schleep seemed to have some little doubt 

 as to whether he should consider this specimen as belonging 

 to a distinct species, or as merely a variety or monstrosity, 

 but is more inclined to believe in the latter solution of the 

 question. He merely describes the outside of the animal, 

 and makes no remarks on the morphological significance of 

 its conformation. 



Yarrell, in the second volume of his "British Fishes," has 

 figured the head of a Brill (Rhombus vulgaris), with a simi- 

 lar condition of the fore-end of the dorsal fin. The upper 

 eye seems to be set nearly on the top of the head. He also 

 mentions having seen similar specimens of the turbot. 



* Note sur la Symmetrie des Poissons Pleuronectes dans la jeune age. Am 

 nales des Sciences Naturelles, 3me series, xx. pp. 340-342. 



