WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 27 
lateral view the pterygo-palatine of its own side. The pterygo-palatine 
(Fig. D) ends abruptly at its posterior end, since the membrane bones 
which are to supersede it in supporting the upper jaw are already de- 
veloped there. 
The left pterygo-palatine (pt-pal. s.) is visible in Figure D only in 
the region between the left ect-ethmoid and the cartilage sphere (ert. 
orb. a.) in front of the ethmoid. This terminal spherical mass of car- 
tilage (ert. orb. a.) can be traced to its position in the adult skull. In 
a fish two inches long the ethmoid cartilage had pushed its way under 
this spherical cartilage, which had elongated in antero-posterior direc- 
tion, but was still located between the nasal pits. I regard it, there- 
fore, as the cartilage which forms in the adult the median anterior por- 
tion of the single orbit in which the left eye is to be found. The nasal 
bones lie on either side of it, and the rest of the orbit is made up of the 
right frontal, the left frontal and the left pre-frontal, or ect-ethmoid, 
bones. 
By comparing the position of the olfactory openings in Figures, B, C, 
and JD, it is plain that there has been a twisting of the ethmoid region 
from left to right, through an are of 90 degrees. The line joining the 
centres of the ect-ethmoids in Figure B is horizontal, whereas in Figure 
C it makes with the horizon an angle of more than 30 degrees, and in 
Figure D is vertical. But with this twisting about the longitudinal 
axis the plane of the ethmoids has also revolved from a transverse 
position into one nearly coinciding with the sagittal plane, — possibly 
due to the pressure caused by the increase in the size of the eyes, — so 
that the axes of the olfactory foramina, which at first were parallel to 
the long axis of the fish, now pass from right to left. Accompanying 
these torsions, there has been a shifting in the relative positions of the 
olfactory foramina and surrounding cartilages till those of the right side 
are considerably in advance of those of the left. Itis, however, the twist 
about the longitudinal axis which makes the migration of the eye seem 
rapid. This occupies in my experience not over three days, and accord- 
ing to Nishikawa (’97) it was completed in the fish which he observed 
in twenty-four hours. 
The whole of the cartilaginous system of the facial region has been 
supported up to this time by two cartilage rods, the fused trabecule 
cranii (trb., Figures A-D; Plate 1, Fig. 7; Plate 2, Fig. 10; Plate 3, 
Fig. 17) and the right supraorbital bar (¢trd. sw’orb. dx., Figures A-D ; 
Plate 2, Fig. 10; Plate 4, Fig. 18). 
The twisting is greatest in the optic region, the brain case showing 
