DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 271 
which bears the vomerine teeth (Plate XXIX. fig. 2) does not participate much in this 
apparent twisting. 
Nasal (is). This bone is tolerably symmetrical as regards the development of its two 
sides. In front it presents a deep transverse notch, at which point the bone is slightly 
bent, the concavity being towards the ocular side. We have then the nasal prominence 
divided into two parts by the aforesaid notch, the upper (?, figs. 5 & 6, Plate X XIX.) 
giving attachment to ligaments connected with the maxillary bones, and continuing 
forwards the direction of the still oblique morphological middle line; the lower and 
anterior (k) forming an articular ridge, on which the intermaxillary nodule of cartilage 
glides downwards and upwards, and forwards and backwards, as the jaws open and 
shut, coincides in its direction with the pseudomesial line also; so that the notch be- 
tween these two prominences of the nasal bone is the point where the morphological 
middle line again returns to the apparent middle line of the top of the head (see 
Plate XXIX. fig. 1). 
The two prefrontals are at once known by the notch borne by each, and which is com- 
pleted into an olfactory foramen (c) by the contiguous nasal bone. 
The prefrontal of the eyed side (1) is somewhat triangular in shape; anteriorly and 
to the inner side it presents the notch already spoken of, articulates with the nasal bone, 
and touches the primordial cartilage. Below the olfactory notch it is extensively con- 
tinuous with the primordial cartilage, and also sends down a process (c) which articulates 
with the corresponding ala of the vomer. Opposite the olfactory notch, and on the outer 
side of the bone, is a prominence (e) to which the anterior suborbital bone of that side is 
attached; and posteriorly a pointed process (a) is sent back, which articulates with the 
interocular process of the corresponding frontal bone, and rests internally on the primor- 
dial cartilage which forms the anterior part of the “ septum iuterorbitale " (see p. 266). 
The prefrontal of the eyeless side (1) is much larger, and of a rudely quadrangular 
shape. Anteriorly it is similarly related to the nasal bone, and presents the same olfactory 
notch that we saw in the other bone. The same process (c) is sent downwards and for- 
Wards to articulate with the vomer; and an additional one (b) is sent downwards and 
backwards to artieulate with the basi-presphenoid. But the great mass of the bone 
Projects backwards in a great flat quadrangular process (f), which, instead of articu- 
lating with the interocular process of the frontal of the eyeless side, as the posteriorly 
directed process (a) of the other prefrontal does with its corresponding frontal, passes 
found the other side of the orbit, and, joining the external angular process of the frontal 
of the eyeless side, forms, with it, the pseudomesial bar of the cranium, which bounds 
the orbit on the eyeless side. 
The orbit which contains the upper eye, then, 
side by the interocular process of the frontal of the eyeless 
* Small portion of primordial cartilage (s), on the inner 
E of the frontal of the eyeless side and by the post 
corresponding prefrontal. 
If we now examine the prefrontal bones in the Co 
VOL. XXV. 
is bounded posteriorly and on the outer 
side, at the anterior angle by 
side by the external angular 
eriorly directed process (^) of 
d, the lateral aspect of one of which 
2Q 
