185 



with tlie ethmoidal cartilage, and practically they arise 

 from the lower portions of the median frontal ridges. 



ISTow if we suppose that the asymmetrical skull of the 

 Plaice first began to appear in an ancestral form 

 resembling the Cod, grave difficulties at once arise. For 

 if a rotation from left to right of the orbital region of such 

 a cranium took place, it is evident that although the left 

 eye might ultimately look upwards, the right on the other 

 hand must also move and would tend to become buried 

 in the tissues of the head. Moreover it is probable that if 

 a round fish such as the Cod adopted sedentary habits on 

 the sea bottom, the flattening, if it occurred at all, would 

 be a dorso-ventral one, as in the case of the skate. 



But if we assume that a laterally compressed fish (like 

 Zews, for instance) took to a bottom habit, and began to 

 lie on one side of its body, the changes necessary to bring 

 about such asymmetry as we find in the Plaice would be 

 much less violent. In such a form we may suppose that, 

 following the lateral compression of the body, the eyes 

 would have moved to near the dorsal edge of the head, 

 whilst the frontals would have become greatly compressed 

 from side to side and probably elongated dorso-ventrally, 

 like the right frontal of the Plaice. The eyes therefore 

 being now close together near the dorsal median line of 

 the head, require to travel so much the lesser distance. 



Now when such a fish assumed a bottom living habit, 

 lying on (say) the left side of its body, any variations in 

 the position of the left eye bringing it nearer the middle 

 line than the right {i.e., nearer the upper side) would be 

 of great advantage. This approximation to the middle 

 line would be attained either by the attenuation of the left 

 frontal or by a shifting of both frontals towards the right 

 side. In the skull of the Plaice both these things have 

 happened. 



