DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDA, 283 
frontal bone (eyeless side), till it ends in front of the orbit, and to the inner side of the 
nasal fossa of the eyeless side, in a “ turbinal ” ossicle( »), which is longer than its fellow 
of the opposite side. 
We have thus the main stem of the mucus-canal of the eyeless side also following the 
morphological middle line of the top of the cranium, crossing the pseudomesial line 
beneath the cephalie part of the dorsal fin, and beneath the supratemporal canals of both 
sides, and passing between the eyes side by side with its fellow of the opposite side, and 
with which, as in the Cod, it is connected by a transverse commissure. This arrangement 
we may also regard as additional evidence that the “ interocular ” bar in the Pleuronect 
cranium is the only and entire homologue of the arch of the frontal bone in the Cod and 
other fishes. 
But where are the suborbital canal and chain of bones of the side under consideration ? 
We have seen that a mucus-canal, contained in a range of minute bones (4 4”), lies along- 
side the upper eye; but we have also seen that this is the supratemporal branch of the 
eyed side. We must accordingly look for some other. From the point behind the orbit 
where the main stem of the eyeless side gives off its commissural branch, is given off in 
the opposite direction a branch (d d) running at first a little backwards, till it emerges 
from the frontal bone, when it turns forwards and proceeds, in the skin of the eyeless cheek, 
pretty closely alongside the pseudomesial bar of the cranium, till it ends in the outer 
side of the nostril of the eyeless side. This canal, enclosed in seven tubular ossicles (7’), 
of which the anterior one is largest, is undoubtedly the suborbital of the side on which it 
is found, and that which should appertain to the upper eye of the flatfish, but situated 
on the other side of the head from that on which its eye is now found ; and not only so, 
but between it and its eye we find the pseudomesial bar of the cranium, the cephalic 
extremity of the dorsal fin, and the supratemporal canals and ranges of bones of both 
sides. In fact, the one eye has passed over to the now binocular side of the fish, leaving 
its suborbital range behind it, while the other structures before mentioned have got 
interpolated between the dorsal fin and the supratemporal canals proceeding forwards 
from behind, the pseudomesial bar being formed partly by a process sent back from the 
prefrontal, and by one sent forward from the frontal bone of the now eyeless side. It 
must be observed, however, that this suborbital eanal of the eyeless side, though it has 
not followed its eye completely round, is yet situated much higher on the side of the 
head than its fellow opposite; indeed, as far as it is concerned, the turning-process has 
Proceeded so far, and then become arrested. 
In the Halibut, in which I have examined these canals with some care, the arrange- 
„ent is much the same, We find the main canal of each side curving round between 
eyes, one contained in the interocular process of each frontal bone, in the nd 
Manner as is represented in the diagram of the Turbot (Plate XXXII. fig. 2). ee 
“i the eyeless side, however, here pierces also the nasal bone, as gum sA e 
© openings in the bone x (Plate XXIX. fig. 10), thus confirming wha ave 
y Stated (page 274), that that part of the nasal bone entering into the boundary of 
- orbit in the Halibut and Plaice is a development from the part of the bone apper- 
to the eyeless side. | 
