DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 273 
In the group of flatfishes to which the Flounders belong, we find the rays of the 
dorsal fin advaneing only so far as the middle of the orbit; and the eyes are normally 
placed on the right side of the head. There is also a very marked tendency for the 
mouth to become twisted towards the opposite side of the body to that on which the 
eyes are placed. 
As characteristic of this group, we first consider the cranium of the Halibut ( Hippo- 
glossus vulgaris) (Plate XXIX. figs. 8-11). 
On looking at the under surface of the Halibut’s cranium (Plate XXIX. fig. 9), we 
find the basal keel pretty straight in itself; but when the head is i» situ on the end of 
the vertebral column, this keel points strongly to the eyeless side. In the occipital 
region the skull is apparently broader on the eyed than on the eyeless side; this is due 
to a greater prominence on that side of the mastoid (s) and exoceipital (2) bones; the 
petrous (1) is also larger. The middle line of the posterior aspect of the skull is also 
strongly eurved, the convexity being towards the ocular side, and corresponds with a 
similar curve, to be afterwards alluded to, in the spinous processes of the anterior ver- 
tebræ (p. 285). 
The basioccipital (1) is unsymmetrical, the middle line of the inferior surface pointing 
to the eyeless side, while that of the upper surface of the bone, indicating the twist of the 
cranial cavity, diverges towards the eyed side. The mesial vertical plane of the bone is 
therefore pushed over to the eyed side anteriorly. 
The basi-presphenoid (s) presents in a much more exaggerated form than in the 
Turbot the apparent twisting-up of its anterior part on its long axis towards the eye- 
less side; here, indeed, the groove in which the end of the vomer is inserted looks 
quite to that side. The axis of the entire bone points to the eyeless side, as I have 
already noticed. 
The postfrontal of the eyed side (2) has the semilunar excavation, which, by a similar 
one in the alisphenoid bone, is completed into an articular cavity (D) for the head of the 
epitympanie, placed further forwards on its surface than on the eyeless side, so that the 
attachment of the suspensory apparatus to the cranium reaches further forwards on the 
former than on the latter side. This is important in connexion with the conformation 
of the bones of the face (p. 278). : 
The orbitosphenoid (0) is larger on the eyeless than on the eyed side, and its long 
‚ Axis points considerably over towards the eyed side. In the view of the under surface 
of the skull given (Plate XXIX. fig. 9) the real size of this bone is not apparent, owing 
to its concealment by the basi-presphenoid. 
The supraoeeipital (3) shows in a more m 
ne. of the morphological from the apparent middle line at 
ts direction is indicated by the red line in the figure. 
im parietal of the eyeless side (7) is broader than the op 
“ver, is often a little longer. 
b^ ontal of the eyed side (1) corresponds very mu 
he Turbot. But that of the eyeless side (7) has its ext 
arked manner than in the Turbot the di- 
the back of the head. 
posite one, which latter, 
ch in shape with the same bone 
ernal angular process much 
202 
