WILLIAMS: MIGRATION OF EYE IN rSEUDOPLEUEONECTES. 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 (crt. 

 orb. a.) in front of the ethmoid. This terminal spherical mass of car- 

 tilage {crt. 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, 

 liones. 



By comparing the position of the olfactory openings in Figures, B, G, 

 and D, it is plain that there has been a twisting of the ethmoid region 

 from left to right, through an arc 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. It is, 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 wliole of the cartilaginous system of the facial region has been 

 supported up to this time by two cartilage rods, the fused trabeculse 

 cranii {trb., Figures A-D ; Plate 1, Fig. 7 ; Plate 2, Fig. 10; Plate 3, 

 Fig. 17) and the right supraorbital bar {trb. su'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 



