276 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDÆ. 
in the same direction as the articular ridge above on the nasal bone. In consequence 
of this conformation of the nasal bone and vomer the long axis of the oral appa- 
ratus points obliquely to the eyeless side, and when the mouth opens it is protruded in 
the same direction. (See description of facial bones.) 
The cranium of the Plaice, then, is more unsymmetrical than that of the Halibut in 
the almost complete non-development of the interocular pro- Fig. 5. 
cess of the frontal bone of the eyeless side; and, in conse- "Fone. 
quence, the corresponding process of the other frontal forms 
almost the whole of the external or lower boundary of the 
orbit. The nasal bone enters more largely into the boundary 
` of the orbit in front, and the process of twisting of the ante- 
rior part of the skull upwards towards the eyeless and down- 
wards towards the eyed side, as well as the bending of the 
axis of the keel of the cranium towards the eyeless side, has 
proceeded to a greater extent. In the adjoining diagram the dotted line represents the 
apparent middle line of the head of a Plaice, the thick black line the axis of the keel of 
the cranium, and the thin black line the morphological middle line. 
The changes from the symmetrical type which have taken place in the cranium of the 
Pleuronectidæ may be summed up in the following propositions :— 
1. The mesial vertical plane of the cranium has become inclined over to the now bin- 
ocular side of the head, very slightly in the posterior part of the cranium, very much in 
the region of the eyes (so that the original vertical interorbital septum becomes now 
nearly horizontal), returning in the nasal region nearly to its original vertical position 
in the Turbot, but never doing so in the Halibut or Plaice. 
2. In consequence of this, the middle line of the base of the skull remains still com- 
paratively straight; while the middle line of the upper surface, diverging from the 
apparent or pseudomesial line, curves round between the eyes (which the turning-over 
of the mesial plane has of course brought to one side), and returns to the middle in 
front. Having got in front of the eyes and nasal fossæ in the Turbot, it again coin- 
cides, or nearly so, with the apparent middle line; but in the Halibut, and still more 
so in the Plaice, the apparent and morphological middle lines, if produced, would cross 
each other. 
3. In the anterior part of the cranium, the parts on the eyeless side of the middle line 
of the base are, in all the Pleuronectidæ, more developed than on the ocular side. This 
is exemplified in the more strong development of the eyeless side of the anterior part of 
the basi-presphenoid, in the greater breadth of the ala of the vomer on that side, in the 
greater breadth of the orbitosphenoid, and in the great development of the processes 
(c and 3 in the figures) sent down by the prefrontal to articulate with the vomer and bast- 
presphenoid. While, on the ocular side, the orbitosphenoid is narrower, the ala of the 
vomer is smaller, and the prefrontal does not articulate at all with the basi-presphe- 
noid. Not only are those parts, on the eyeless side of the middle line below, more der 
e ei ocular side, but their development is in a more vertical do 
; e whole anterior part of the cranium assumes an appearance 88 if 1 
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