SEA-FISHERIES LABORATORY. 331 



the riglit prefrontal retained its connection with, the right 

 frontal, but lost its connection with the parasphenoid bar. 

 After the rotation of the orbital region the left prefrontal 

 grew backwards and the left frontal forwards, so as to 

 form the secondary junction of these two bones already 

 referred to. 



(2.) The downward shifting of the oblique eye 

 muscles from practically the frontal to the parasphenoid 

 bar. This possibly began in the symmetrical but laterally 

 flattened ancestor, as the frontals became attenuated from 

 side to side and deepened dorso-ventrally. But it was 

 continued in the asymmetrical fish. The object of the 

 shifting of the eyes was to make dorsal vision possible 

 with both eyes. In the Cod the origins of the oblique 

 muscles are best adapted for lateral vision and for visual 

 axes in approximately the same straight line. In the 

 Plaice, however, the visual axes are directed dorsally, and 

 to bring this about the oblique muscles had to shift 

 ventrally so as to exercise a more effective pull over the 

 eyes. The origins of these muscles therefore moved down- 

 wards towards the ventral region of the cranium and 

 ultimately to the parasphenoid bar. 



(3.) The attachment of all the oblique muscles to the 

 left prefrontal. We have already pointed out that the 

 right prefrontal has lost its connection with the 

 parasphenoid bar. This would doubtless have occurred in 

 the earlier stages of the rotation of the eyes. When, 

 therefore, in the final stages of the torsion, the oblique 

 muscles in their downward passage over the interorbital 

 septum arrived at the bar, the right prefrontal must have 

 already left it. They thereupon continued their migra- 

 tion on to the left side and became attached to the left 

 prefrontal — the only convenient attachment remaining. 



(4.) The passage of the right oblique muscles through 



