270 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 
the interocular processes of the frontal bones, till they end in the nostrils and front of the 
snout. 
The main slime-canals, one for either side, which in the Cod pass between the eyes 
over the frontal arch, here pass between the eyes along the interocular bar, being chan- 
nelled out in the interocular process of its corresponding frontal bone. (See Part III. 
of this essay.) 
But what, then, is the pseudomesial bar or bridge in the Turbot's cranium, if the inter- 
ocular bar be the complete representative of the frontal arch in the Cod ? This we shall 
see presently. 
We now come to the anterior or nasal part of the cranium, characterized by two 
olfaetory foramina, of which that of the ocular side is more anterior than the other. As 
in the Cod, this portion of the cranium consists of a central piece of cartilage, support- 
ing four bones, the vomer below, the nasal above, and the two prefrontals, one on each 
side. 
The Cartilage.—This portion of cartilage (A, figs. 1-5), more extensive than the 
corresponding remnant in the Cod, appears as a very rudely quadrangular plate with a 
large hole through its middle, connected to bones all round its edges, save the concave 
posterior one, whieh is continuous with the fibrous septum between the eyes, already 
referred to. On each side of it an olfactory nerve passes to its corresponding olfactory 
foramen in the direction indicated by the bristles in Plate X XIX. figs. 3-5. It there- 
fore indicates the morphological mesial plane of the anterior part of the cranium, and 
would be vertical were it not for the twisting over to one side which has occurred. 1t 
rests beneath on the basi-presphenoid and vomer; in front it supports the nasal bone 
(15), and laterally, round its anterior inferior angle, it is intimately connected to the 
prefrontals (14&14), one on each side. Above, the anterior extremities of the inter- 
ocular processes of the frontal bones rest on it, as follows :—A longitudinal notch divides 
the upper edge of the cartilage into two unequal pointed processes (g & h, Plate XXIX. 
figs. 3-5). Of these, that of the eyed side (g), by far the largest, is lodged in a hollow 
on the under surface of the stout interocular process of the frontal of the same side, and 
supports also the posteriorly directed process (a) of the corresponding prefrontal (14); 
the other one (4) is similarly related to the end of the slender interocular process of the 
frontal of the eyeless side, but is not touched by its prefrontal, save at its very base. 
Now this notch, separating those two processes, as it indicates the line of separation 
between the interocular processes of the two frontal bones, must likewise indicate the 
morphological middle line of the cartilage; so that here we have a mesial cartilage, not 
only unsymmetrical in its position, but also in the development of its two sides, the 
greater development being on the ocular side of the fish. 
Vomer (13). The posterior part of the vomer, which fits into a groove on the lower 
aspect of the basi-presphenoid, is more developed on the eyeless than on the eyed side; 
on the eyeless side also the ala for articulating with the prefrontal is larger, and projects 
more vertically upwards than on the opposite side; so that, like the anterior part of the 
basi-presphenoid (p. 265), the vomer has slightly the appearance of being twisted up e 
its long axis towards the eyeless side. In outward form, however, the head of the bone 
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