DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDA. 279 
the opercular bones on the eyeless side, in the Turbot and the Brill, is the only eireum- 
stance I know of in the osteology of the face in the flatfishes which shows the least 
discrepancy with the principles laid down. 
Pterygo-palatine Apparatus.—Corresponding with ‚the general flattened form of the 
whole fish on the eyeless side, and because the cheek of that side has no eye to accommo- 
date, the palate, entopterygoid, and ectoptergoid bones are considerably flatter on the eye- 
less side; while on the opposite side, to form the floor of a sort of orbit for the lower eye, 
the entopterygoid bone must arch considerably inwards. In the Turbot and Brill the 
articulation of the lower jaw to the suspensory apparatus is on much the same level 
on both sides; but we have seen that in all the Pleuronectidæ the attachment of the 
palate-bone to the cranium is higher on the eyeless side; therefore the palate and ec- 
topterygoid bones, having more space to traverse on that side, are considerably larger. 
They are also much stouter than on the ocular side. The entopterygoid bones of both 
sides are about the same length; but that of the ocular side is rather the broader of 
the two. 
In Plate XXX. figs. 1, 2, the palato-suspensory and opereular apparatus of each 
side of Rhombus maximus are figured, the bones being numbered according to the list 
given at the end of this essay. 
In the Halibut (Hippoglossus vulgaris) the facial bones are asymmetrical on the same 
principles as those which operate in the case of the Turbot above described; but two 
additional eireumstances connected with the jaws exaggerate that asymmetry very con- 
siderably. 
l. The mouth seems to be twisted on its own axis, so that not only is the articulation 
of the palate-bone to the cranium higher on the eyeless side, but so is also the articulation 
of the lower jaw to the hypotympanic. (See Plate XXX. fig. 3, where the end of the 
cranium, with the attached palato-suspensory apparatus of each side, is seen directly 
from the front.) 
2. The mouth does not open straight forwards, but when the fish gapes it points ob- 
liquely towards the eyeless side, the upper jaw-bones, when the mouth opens and shuts, 
ding downwards and forwards, upwards and backwards, on the oblique ridge on the 
tasal bone and the oblique facets on the vomer already described (see p. 274). 
Accordingly we find the intermaxillary and maxillary bones a little stouter on the 
eyeless side; the maxillary of the eyeless side is flatter, and has the tubercle for attach- 
1 of the tendon of the retractor maxille muscle much larger, and situated lower 
xn than in the bone on the other side; the convex facet on the head of the bone for 
Sang on the vomer is likewise larger. The lower jaw is a little longer, and considez- 
^y More arched on the eyeless side ; its dentary bone is likewise armed on this side 
with a greater number of teeth. ; : 
hs ; ensory and Opercular Apparatus.—As in the Turbot and Brill, the epitympanic 
one is larger on the eyed side; and the disproportion between the bones of the two 
| SMA greater, because the articulation of the lower jaw is lower down on Fa 
a Dur Side. And because that; articulation is also further forwards on the .. j > 
Vor, dua mesotympanic, pretympanic, and hypotympanic longer on "e = 
