DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ 277 
were twisted, up to the eyeless side, down towards the ocular side. In connexion with this, 
we must notice the greater elevation on the eyeless side of the olfactory foramen, and of 
the articulation to the cranium of the palatine apparatus and of the anterior suborbital 
bone. 
4. On the top of the head the interocular parts of the frontal and prefrontal bones are 
more developed on the ocular side. The interocular process of the frontal of the ocular 
side is always much stouter than that of the other bone, and always articulates with a 
corresponding process sent back from the prefrontal. But the prefrontal of the eyeless 
side sends back no process to articulate with the frontal of the same side, whose inter- 
ocular part, if examined in a series of flatfishes, gets smaller and smaller, till in the 
Plaice it seems almost gone. The same condition affects the morphologically mesial 
plate of cartilage forming the anterior part of the interocular septum, which cartilage 
we have already seen to be chiefly developed on the ocular side. 
5. To accommodate the two eyes, now both on one side of the head, the anterior parts 
of the frontal bones remain as a narrow bar, never widening out into a broad arch as in 
the Cod and other fishes. Accordingly, to maintain the requisite stability of the cra- 
nium, a new bar or bridge of bone is formed (pseudomesial) by the union of a process 
sent forwards from the anterior external angle of the frontal of the eyeless side with one 
sent back from the corresponding prefrontal. By means of this bar the upper eye 
becomes closed round by a bony orbit, whose boundaries in the Turbot consist of the 
interoeular process of the frontal of the eyeless side, the external angular process of the 
same bone, the external angular process of the corresponding prefrontal, and a small 
portion of cartilage in front. In the Halibut and Plaice, however, the nasal bone comes 
to take part in the boundary of the orbit principally by a development from its eyeless 
side; and in the latter fish, owing to the atrophy of the interocular portion of the frontal 
of the eyeless side, the corresponding part of the other frontal forms almost the entire 
external boundary of the orbit. 
6. The olfactory foramen and the place of suspension of the anterior suborbital bone are 
further forward on the ocular side, slightly in the Turbot, to a marked degree in the 
Halibut and Plaice, in which latter fish the entire prefrontal bone is on this side 
E further forwards. The articulation of the epitympanic bone to the cranium, in 
e Halibut and Plaice, likewise extends further forward on the ocular side. 
7. The axis of the keel of the cranium, pretty straight in the Turbot, points, e 
E and still more so in tbe Plaice, to the eyeless side. u ee 
m and, it 38 bent with the convexity downwards—a condition app y 
e peculiar mechanism of the jaws in that fish. 
in 
II. Bones of the Face. 
Janes, Palato-suspensory Apparatus. | Opercular Apparatus. 
ti Pleuronectidæ are also unsymmetrical, but in a much less degree th SE 
cranium. Before proceeding to study their asymmetries, however, we must take into 
the following circumstances, which seem to act as the conditions on which these 
__The bones of the face 
an those of 
