288 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 
middle line as the fin itself, it is hardly possible to imagine that middle line and one eye 
migrating beneath the superimposed parts without the symmetry of the latter being 
affected, had they been in their present position at the time that the supposed migration 
took place. ; 
2. All those parts accompanying the dorsal fin in the head show traces of having come 
from behind. That part of the lateral or body-muscle, which lies on the top of the head 
consists of the anteriorly reflected portions of muscle-segments posterior to the cranium. 
The nerves supplying the fin-rays and muscles in this region are derived from the dorsal 
branches of the first three or four spinal nerves, which turn forwards over the head from 
behind. The supratemporal mucus-canals and bones, which, commencing posteriorly, also 
proceed forwards in defiance of the morphological middle line, are supplied by branches 
of the vagus nerve, which of course also turn round from the back of the skull and run 
forwards. Lastly, the blood-vessels supplying the fin-rays, &c., on the top of the head 
accompany the branches of spinal nerves already mentioned, and proceed to their desti- 
nation from behind forwards. 
3. The embryological observations of Van Beneden seem to favour the same view; and 
I have already referred (p. 264) to the paper wherein he describes a young Pleuronect 
in which the two eyes were still one on each side, and the dorsal fin only yet descended 
to the middle of the cranium. To this observation I may add one of my own. In the 
summer of 1863 I obtained, in dredging over a sandy bottom in the Frith of Forth, three 
young Pleuronectidæ, each about half an inch long, and apparently belonging to the 
genus Platessa. In two of them the eyes and dorsal fin were conformed as in an adult 
Flounder, but in the third (the anterior part of which is figured, Plate XXXII. fig. 9, 
magnified five diameters) one eye was nearly on the middle line, with, as in M. van 
Beneden's specimen *, the dorsal fin stopping short behind it. 
4. The structure of certain malformations occasionally found amongst flatfishes also 
confirms the view I have adopted. "These monstrosities are specimens of Pleuronectidæ 
where the upper eye is found more towards the top of the head than usual, the dorsal fin 
not being fixed down by the side of it, but projecting above it, supported on a free pointed 
process. Many of these specimens have been already recordedt. The most remarkable 
case I am acquainted with is one described and figured by Schleep $, occurring in à 
Turbot, which he calls Pleuronectes maximus duplex, thinking that possibly it might be 
a distinct species. Here the two eyes are still one on each side of the head; the right 
one, however, is higher than the left, and seems just about to make the turn, while the 
anterior part of the dorsal fin projects over the eyes supported on a free pointed process. 
I have seen cases of this condition both in the Turbot and Flounder (Platessa flesus), 
though not in so exaggerated a degree as in Schleep’s specimen. In Plate XXXIL 
figs. 8-9, are represented both sides of the head of such a specimen of P. flesus, the mucus- 
canals being indicated by the shading in horizontal lines. The upper or left eye is see? 
to be situated on the top of the head, having, we may say, just turned the corner and n0 
more; while the dorsal fin presents the appearance already referred to. Here also, on the 
a 3 à 
Loc. cit. + See Yarrell’s * British Fishes,’ vol. it. 
+ Oken’s ‘ Isis,’ 1829, S. 1049. 
