272 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDA. 
is figured in Plate XXIX. fig. 16 (1), and disarticulated in fig. 17, we shall find that the 
connexions of each bone are as follows :— 
1. It shows two processes (dd) going towards the nasal bone, with an olfactory notch 
between them (c). 
2. An anteriorly directed process (c) for the ala of the vomer. 
3. A posterior-inferior process (5) for articulation with the basi-presphenoid. 
4. A posterior-superior process (7) going upwards and backwards to join the frontal bone. 
5. Alateral process (e), tipped with cartilage, opposite the olfactory notch, to which the 
anterior suborbital bone is attached. 
Now, on comparing the prefrontal of the ocular side in the Turbot with this, we find 
that everything corresponds exactly, save that the process (b) for articulation with the 
basi-presphenoid is wanting, the interval being filled up by mere cartilage. (Plate 
XXIX. figs. 2, 4, 5.) 
The prefrontal of the eyeless side, though it presents a large process (5) for articula- 
tion with the basi-presphenoid, shows no trace of the process («) for articulating with 
the interocular part of the frontal; it does not touch it at all. But what, then, is the large 
process f? That it is not homologous with the process (v) projecting upwards and 
backwards in the Cod and in the other prefrontal of the Turbot, is evident from its bearing 
no relation to the olfactory nerve of its side, nor to the interocular septum. On the other 
side, the olfactory nerve runs close beneath the process a, as indicated by the bristle 
in Plate XXIX. fig. 4 It follows, then, that this process (f) in the prefrontal bone 
of the eyeless side is an additional process having no homologue either in the Cod or in 
the prefrontal of the opposite side in the Turbot. We may call it “ external angular," 
corresponding with that process already described in the frontal of the same side, and 
which has also no homologue in the Cod or in the eyed side of the Turbot. 
And now we see what the nature of that bar of bone is, which I have called pseudomesial 
(p. 267), and which one is apt at first to think homologous with the whole or part of the 
frontal arch in the Cod and other symmetrical osseous fishes. Seeing that the true 
homologue of the frontal arch in the Cod’s head has been reduced to a narrow bar, and 
twisted over to one side (p. 269), we have, in the pseudomesial bar, a secondary forma- 
tion destined to supply the place of the weak and displaced frontal arch in forming à 
strong and efficient bridge of connexion between the anterior and posterior parts of the 
cranium, and also to support the cephalic continuation of the dorsal fin. 
The cranium we have just considered is the least asymmetrical and most easily un- 
derstood which I have met with in the Pleuronectidæ. We shall now proceed to exa- 
mine and compare with it the crania of some of the other Pleuronectidæ, and note to 
what further steps the process of distortion proceeds, before finally generalizing on the 
changes which have taken place. | 
The cranium of the Brill (Rhombus vulgaris) is nearly identical with that of the Tur- 
bot. But we must remark that the interocular process of the frontal of the eyeless side 15 
proportionally more slender than in the Turbot, while the external angular process s 
the same bone is more pronounced, and forms more of the inner wall of the orbit, than 
in the last-named fish. 
