330 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



It is very remarkable that comparatively slight 

 changes would have sufficed to bring about the distortion 

 of the Plaice's skull starting with such conditions and 

 tendencies as we have indicated above. In the head of 

 the Plaice the orbital region only has suffered extensive 

 change. The otic region has undergone practically no 

 change, and the prefrontal region certainly less than the 

 orbital. The jaw apparatus is not affected at all by the 

 torsion of the orbit, but has an asymmetry of its own (as 

 described above), and this latter doubtless explains the 

 asymmetry of the prefrontal region, which is of a different 

 character to that of the orbital. We have, therefore, now 

 to consider whether such relationships in the anatomy of 

 the skull and eyes as obtain in the Plaice can be reason- 

 ably expected to have followed from such a course of 

 evolution as we have sketched above. There appear to us 

 to be four difficulties requiring explanation. These are 

 as follows : — 



(1.) The relations of the two prefrontals. As the 

 right frontal bent over further to the right side, the right 

 eye would be forced downwards. To make room for it 

 the parasphenoid bar most probably bent over to the left 

 to the position in which we now actually find it. The pre- 

 frontal region lagged behind in this shifting, being 

 unaffected except as regards the movements of the adja- 

 cent parts (orbital region and jaw apparatus). If a Cod's 

 skull be examined it will be seen that the abortion of the 

 left frontal would result in the separation of the left 

 prefrontal from the remainder of the frontal area, and 

 further that the greater rotation of the frontal over the 

 prefrontal area would result either in the separation of the 

 right prefrontal from the right frontal, or in its separation 

 from the parasphenoid bar. Now the latter having itself 

 rotated to the left, we therefore find that in the Plaice 



