268 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDE. 
and accordingly on the outside of the skull covers up more of the mastoid and exoceipital 
bones from view. 
The Postfrontal (12) is longer on the eyeless side, and its long axis curves a little 
round to the eyed side. (See fig. 3, Plate XXIX.) 
Parietals (7). As in the case of the postfrontals, the parietal of the eyeless side is 
considerably longer than its fellow. The difference is best seen when the bones are 
disarticulated. 
The bones enumerated in the last paragraph show but little in regard to the symmetry 
of the head. In those next to be noticed, the indieations are more decided. 
Basi-presphenoid (5). This bone is slightly bent towards the eyeless side, a little 
behind its middle. For description it may be divided into three parts. The posterior 
flattened part, which overlaps the basioccipital, is symmetrical. The middle part presents 
above on each side an ascending laminar process or wing, which bounds laterally a 
channel lodging the origin of the eye-muscles, this channel, however, passing obliquely 
across the long axis of the bone from the eyeless towards the eyed side. The anterior 
part, whieh receives the pointed end of the vomer, is apparently twisted on its long 
axis up towards the eyeless side, this appearance being principally caused by a greater 
development in a more vertical direction of that side of the bone. 
Orbitosphenoid (10). On the eyeless side this bone is longer than on the eyed side; the 
direction of its axis also agrees with that of the eye-muscle canal of the basi-presphenoid 
in pointing obliquely across to the eyed side (see Plate XXIX. fig. 3), the part shaded 
with horizontal lines indicating the cartilaginous tips of the bones. 
The Supraoceipital (3) presents a scale-shaped “body,” forming part of the roof of 
the cranial cavity, surmounted by a very prominent ridge or spine. The flattened part 
is tolerably symmetrical; its long axis, often slightly curved, points, however, to the 
eyed side; but the spine (8), though commencing posteriorly in the middle line, passes 
forwards, with a slight deviation towards the eyeless side, impinges on the frontal bone 
of that side, and, if continued further forwards, would pass quite by the eyeless side of 
the orbit. Its direction, though thus slightly deviating, is, however, nearly in the midde 
line of the top of the head. 
The supraoccipital bone is thus very unequally divided, the larger moiety being on 
the ocular side. 
If now, before we proceed further, we turn to Plate XXIX. figs. 2 and 3, we shall see 
that, although the basal keel of the cranium is continued forwards in nearly a straight 
line, the long axis of the cranial cavity, which that keel underlies, points round to the 
eyed side, anteriorly crossing that keel at an angle. "This is well illustrated by the 
structure of the basi-presphenoid bone itself (5) where the eye-muscle channel, which 
Is simply the lower part of the cranial cavity, crosses the long axis of the bone. This 
also explains the reason why several of the cranial bones, as the parietal, mastoid, post- 
frontal, and orbitosphenoid, are longer on the eyeless side, simply because they have 3 
longer extent to traverse, as is illustrated in the accompanying diagram (fig. 1.) 
CUN that, although the long axis of the supraoccipital bone follows the ger 
st of the cranial cavity in pointing towards the eyed side (indicated by the dott 
