DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 269 
line in the diagram), the spine of the bone continues forwards nearly in HR 
the same straight line as the middle line of the entire fish (indicated by ‘ie | ac 
the straight line in the diagram). To the first of these lines, continuing 
forwards the axis of the body of the supraoceipital bone, I give the name N 
*morphological middle line;" and to the line of the spine of the supra- \ N 
occipital, which also supports the cephalic prolongation of the dorsal fin, the | 4 
name * pseudomesial." 
In advance of the orbitosphenoid, postfrontal, parietal, and supraoccipital bones are 
two bones (11 & 11), which, from their position, must be the frontals. Each consists of a 
posterior somewhat square-shaped part, forming part of the roof of the brain-cavity, and 
of an anterior more slender curved part, which, with its fellow of the opposite side, 
forms the interocular bony ridge or bar. The anterior part of the bone of the ocular 
side (m', Plate XXIX. figs. 1 & 7) is strongly curved, the concavity looking upwards 
and to the eyeless side; at its extremity it articulates with the prefrontal of its own side, 
touches the nasal bone, and rests likewise on a portion of primordial cartilage, to be 
presently described (A, fig. 1). The bone of the eyeless side (Plate XXIX., fig. 7, 11) is dis- 
tinguished by having its posterior part larger, the external anterior angle (x) projecting 
forwards so as to take a slight part in the formation of the pseudomesial bar of the 
cranium, which bounds the orbit on the eyeless side. The anterior part (m) (“ inter- 
ocular process") is much more slender than the corresponding part of the bone of the 
ocular side, and to which it is closely applied; at its extremity it likewise rests on a 
portion of primordial cartilage, but does not toueh its corresponding prefrontal. It 
forms the entire lower boundary of the orbit, and, with its fellow of the opposite side, 
forms the interocular bar of the cranium. 
We now see that the morphological mesial line continued from the supraoccipital 
bone in the line separating the two frontal bones from each other, at first deviating but 
slightly towards the eyed side, afterwards curves round the orbit along the interocular 
bar till anteriorly it tends again to coincide with the apparent middle line, E 
as in the adjoining diagram. In the figures of crania (Plate XXIX.) I have also 
“presented the course of the morphological middle line by a dotted line. That S Les 
this is the true morphological middle line, and that the interocular bar is the ~ ( © 
only and complete homologue of the frontal arch in the Cod, is proved simply x 
by the fact that between the interocular bar and the basi-presphenoid bone 
there extends a fibrous membrane, having imbedded in it the olfactory nerves | 
as they proceed to the nasal fossæ in front. This membrane represents the 
a interorbital septum in the Cod; but, by the twisting over of the m plane 
? ocular region to one side, this septum in the Turbot, instead of remaining vertical, 
has become nearly horizontal, and instead of coinciding with the mesial plane of the 
test of the fish, has become set nearly at right angles to it. In the Turbot, as in all the 
they Plouroneetide, the olfactory nerves are not contained in a tubular prolongation of 
im ty at the top of the interorbital septum as m the Cod, but are simply 
Bi ded in its substance. The ophthalmic branches of the fifth nerve on each side, 
"Ih in the Cod lie beneath the frontal arch, here curve round between the eyes, along 
L 
