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DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 287 
places of attachment proceed no further forward than the supraoccipital bone. But the 
anterior one is of considerable length, and, directed forwards, curving also a little down- 
wards, carries the anterior rays of the dorsal fin in front of the eyes and even of the mouth. 
In the Halibut and Plaice there are six to eight of these bones on the top of the skull. 
In their origins they have advanced from the supraoccipital bone on to the frontal of 
the eyeless side; but the anterior one is not so long proportionally as in the Sole, and 
only carries the first rays of the fin to opposite the middle of the upper eye. 
In a specimen of the Brill, I found ten interspinous bones on the upper aspect of the 
head, their cranial attachments advancing over the supraoccipital and over the frontal 
of the eyeless side, till the anterior one takes its origin even from the prefrontal of the 
same side. This anterior interspinous bone carries the first rays of the dorsal fin to 
beyond the eyes, but not so far as in the Sole. ; 
We thus find that in the Brill and in the Sole the dorsal fin has advanced along the 
head further than in the Halibut and Plaice: in the Sole this has been effected by an 
excess of the first method of advance, in the Brill by an excess of the second. The 
direction in which the fin advances is nearly straight forwards, in the same straight line as 
the middle line of the back, inclining only very slightly towards the eyeless side. It thus 
completely disregards the morphological middle line of the top of the head, being sup- 
ported anteriorly on the pseudomesial bar of the cranium, and on the ridge extending 
on to this bar from the centre of the supraoccipital bone. A part of the lateral muscle 
passes on each side on to the top of the head, along with the dorsal fin, and is arranged 
alongside that structure in equal disregard of the morphological middle line, as like- 
wise are the supratemporal canals. 
— Now, of this remarkable circumstance, there are only two explanations possible. Either 
the dorsal fin is in its original morphological position*, and the upper eye has passed 
under it, or the fin has advanced forwards from behind after that eye has turned over 
from the side to which it originally belonged. To the latter view, which is indicated in 
the paper by Van Beneden already quoted, I must, for the following reasons, give in my 
adherence. 
l. The structure of the cranium shows clearly that the transference of the upper eye 
is eonneeted with the deviation, in the ocular region, of the original middle line of the 
top of the head over to the now binocular side, and that the eye in question preserves its 
morphological relations to the frontal bones and the neighbouring structures quite intact, 
the view that it has migrated beneath any of the parts of the skull in the manner held 
by Rosenthal and Steenstrup being quite untenable. Now, the structures accompanying 
the cephalic end of the dorsal fin showing the same disregard of the morphological 
* Comparative Anatomy.” 
* This was Meckel's opinion, as may be gathered from the following extract from his ede — 
Speaking of the interspinous bones on the top of the cranium in the Pleuronectidæ, he says, “ This disposition is 
“tremely interesting ; it helps to establish the analogy of the cranial bones with the vertebra ; these accessory rays 
Pur placed in fact on the occipital and parietal crests in the same way as those 0 
wer spinous processes.”  (Op.cit. French edition, p. 312.) 
“lls passage would indicate that Meckel had quite overlooked the fact that t 
ver the morphological middle line. 
VOL. XXV. 
f the trunk are situated over the 
hose cephalie fin-rays are not placed 
2:8 
