280 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ 
disproportion being, however, least marked in the case of the mesotympanic. But in all 
the three bones there would have been still more disproportion, were it not that the 
articulation of the epitympanic to the cranium extends further forwards on the eyed 
side (see p. 275). And because the articulation of the palate-bone to the eranium is 
further forwards on the ocular side, we have the entopterygoid slightly longer on that side. 
But that articulation of the palate-bone being much higher on the eyeless side, we have 
an increase in length and stoutness of the palate and ectopterygoid bones of the eyeless 
side, which would have been still more marked, were it not that the articulation of the 
lower jaw to the hypotympanic is higher on the side in consideration. 
Opercular Apparatus.—Both on account of the more arched form of the eyed side of 
the head, and because the articulation of the epitympanic advances on this side further 
forwards, so that the opercular bones have more space to cover, we find on the eyed side 
the opereulum and suboperculum larger in every way than their fellows of the eyeless side. 
And because of the more anterior position of the articulation of the lower jaw on the 
ocular side, we find the interoperculum longer on the same side. A combination of these 
two circumstances, together with the fact that the articulation of the lower jaw is 
lower on the eyed side, renders also the preopereular bone of the eyed side larger in every 
way, the increase in length being, however, most marked in its horizontal ramus. 
In Plate XXX. fig. 3, I have figured the cranium and palato-suspensory apparatus of 
the Halibut, seen directly from the front. Observe, on the eyeless side, the more ele- 
vated position of the olfactory foramen, of the attachment of the palate-bone to the 
cranium, and of the trochlear articular surface for the lower jaw, and the general flatness 
of the palato-suspensory apparatus. 
In the Plaice (Platessa vulgaris) the facial bones are construeted and arranged on 
exactly the same principles as those in the Halibut last described, but with some exag- 
geration of the asymmetries. Indeed when a Plaice gapes, its mouth turns round 
towards the eyeless side in a most remarkable manner; and that side being undermost 
when the fish is swimming in its natural position, I suppose it is thereby better enabled 
to piek up from the sea-bottom the small shell-fish, crustacea, and sandstars which are 
always abundantly found in its stomach when opened. The principles on which this is 
effected are the same as those on which the minor degree of the same sort of obliquity 
depends in the Halibut, and may be thus enunciated. 
1. The very oblique direction of the articular ridge on the front of the nasal bone, 0n 
which the cartilage supporting the intermaxillary bones glides. Its direction necessitates 
these bones, when the mouth opens, to move downwards, forwards, and to the eyeless 
2. The great obliquity of the axis of the two facets on the front of the vomer, on which 
the heads of the superior maxillary bones, along with the interposed fibro-cartilaginous 
disks, glide. That of the ocular side looks’ forwards rather than laterally; and a line 
bisecting the angle formed by the two facets would pass obliquely to the eyeless side. 
Ec. y, in their movements, the superior maxillary bones follow the intermaxil- 
laries towards the eyeless side when the mouth opens. 
3. The articulation of the lower jaw to the suspensory apparatus is further forwards 
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LCCOPGING 
