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 tlie deviation of the original 
middle line of the head. Thus we are afforded a ready and 
rational explanation of the difiiculty 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. Van 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 stir la Symmetrie des Poissons Pleuronectes dans la jenne age. An- 
nales des Sciences Naturelles, Sine series, xx, pp. 340-342, 
