DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 289 
eyeless side, the relation of the suborbital canal to the upper eye is very obvious, 
the fin not having become interpolated between them as in the perfectly developed 
flatfish. 
These monstrous Pleuronectide may be accordingly defined as flatfishes in which the 
turning-round of the upper eye to the other side of the fish has been arrested when 
it has got about the middle of the top of the head, and in consequence the passage 
forwards and tying down of the anterior part of the dorsal fin has also been stopped, or 
obviously it would cross over the eye instead of passing by the inner side of it, as in the 
normal flatfish. It accordingly projects upwards and forwards on a pointed process over- 
hanging the eye, as in the specimen figured. It is worthy of remark that all those ab- 
normal specimens are equally coloured on both sides, as if the animal, not having per- 
feetly aequired the characteristics of a flatfish, swam with either of its sides upwards 
and exposed to the light at pleasure. In the case of Turbots affected with this con- 
dition, the bony tubercles also, usually characteristie only of the ocular side, are found 
equally distributed on the eyeless one. 
As we must consider those monstrosities to be dependent on arrested development, the 
only developmental circumstances which we can safely infer from the appearances pre- 
sented are, that the upper eye turns round on the top of the head, and that then the 
dorsal fin advances past it. 
But the young Plagusie figured by Steenstrup would seem to contradiet directly the 
above-advocated theory, and prove that the upper eye gets to its present position by 
. passing beneath the dorsal fin. In some of his specimens the transposition seems never 
to have taken place at all; but the one which seems most fully to justify his views is one 
Where the animal seems to have three eyes, the eye of one side projecting also through 
a little fissure above that of the other side, which side becomes thus binocular. This 
appearance is so striking that one might readily be excused in thinking with M. Steen- 
strup, “Can we imagine a more striking demonstration of the passage of the eye across 
the head than an eye arrested in this position ?" Another specimen described by him 
has an eye on each side of the head, but above the left eye is a little slit where the other 
should appear. 
Although it must at once be acknowledged that these observations are very remark- 
able, and not to be cast aside merely because they do not tally with our preconceived 
theories, yet it seems to me that considerable research is still required before we can 
accept these specimens as representing the normal process of development in all the 
Pleuronectidæ ; for the structure of the head of the adult flatfish seems to me most conclu- 
sively to prove that the upper eye does not pass beneath or through any part of the bony 
cranium, and that the dorsal fin and its associated structures advance from behind, while the 
Structure of the well-known “ monsters," and the observations of Van Beneden and also 
of myself on young Pleuronectidæ, certainly indicate that in the genera Rhombus and 
Platessa, at least, the dorsal fin advances after the upper eye has turned round on the 
top of the head. 
ButM. Steenstrup’s strange specimens certainly open up the question whether there be 
any group of flatfishes in which, in the normal course of development, the “ape fin ex- 
i 28s 
