278 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 
1. The form of the jaws and the direction in which the mouth opens. These conditions 
vary somewhat in the different Pleuronectidean types. In the Turbot and Brill the jaws 
are pretty symmetrically conformed, and the mouth opens nearly straight forwards, as in 
an ordinary fish ; whereas, in the Halibut and in the Flounders, the jaws on the eyeless 
side are considerably stronger and more arched than on the eyed side, and the axis of 
the mouth, in opening, always tends to point towards the eyeless side. In the Sole, on 
the other hand, it is by means of the strange conformation of the jaw-bones of the eye- 
less side that the mouth is rendered chiefly effective on that side, the jaws on the eyed 
side being even (as is well known) perfectly toothless ; and we find, dependent on the pe- 
culiar shape of the jaws, variations in the form of the palato-suspensory apparatus of 
each side, wherein the Sole differs remarkably from the other Pleuronectidæ I have 
examined. 
2. The flattened form of the eyeless side of the fish, and the more arched form of the 
ocular one. 
3. The fact that the cheek of the eyed side must accommodate an eye, while the other 
side has been relieved of its corresponding one. 
4. The greater and more vertical development on the eyeless side of the ala of the 
vomer, and of the corresponding part of the prefrontal; so that the articulation of the 
palate-bone to the cranium is higher on the eyeless than on the eyed side. 
5. The more anterior position of the parts about the olfactory region which belong to 
the oeular side, so that the articulation of the palate-bone to the cranium is further 
forwards on that side. In the Halibut and Plaice this condition affeets parts further 
back (see p. 273), so that the articulation of the suspensory apparatus is also further 
forwards on the eyed side. 
'The symmetries of the bones of the face are not much altered in the Turbot and Brill. 
A greater degree of asymmetry is found in the Halibut and in the Flounders ; while the 
facial bones of the Soles are the most unsymmetrical of all. 
Turbot and Brill.—In the Turbot and Brill the mouth looks nearly straight forwards, 
and, in the movements of opening and shutting, the upper jaw-bones move en masse nearly 
straight forwards and backwards—a cireumstance brought about by the nearly straight 
back-and-forward direction of the ridge on the nasal bone for the intermaxillary carti- 
lage, and by the symmetrical position of the facets on the vomer for the heads of the 
superior maxillary bones. "The intermaxillary and maxillary bones are very nearly alike 
in size on the two sides; the intermaxillary of the ocular side is a little longer, more 
arched, and furnished with more teeth than its fellow. "The head of the superior maxil- 
lary bone of the ocular side has a smaller articular facet for gliding on the vomer. The 
lower jaw is longer, and somewhat stouter on the eyeless side; the dentition is much 
the samo on both suspensory and opercular apparatus. In the Brill we generally find 
| = ~ epitympanic and preopercular bones are slightly longer on the ocular side; 1n 
the Turbot they are very nearly equal. But in both the Turbot and Brill the operculum 
"s n kis tet dg L larger on the eyeless side; the interoperculum is also ym 
the. “eut z gi «prn shorter than on the ocular side, because, on the latter 8! i 
e à tion of the lower jaw is further forwards. The slightly greater breadth 0 
