EVOLUTION OF ASYMMETRY ii 



whether composed of membrane, cartilage, or a thin lamina of bone, has very much 

 the same relation to the eyes as in symmetrical fishes, but has come to lie in a morpho- 

 logically horizontal instead of in a vertical plane. Apart from the reduction in the 

 extent to which the frontal bone of the bUnd side contributes to the formation of 

 the interorbital bar in the more specialised forms, the relations of the two frontals 

 with the eyes is constant throughout the Heterosomata, although in such forms as 

 Bothus and Engyprosopon, in which the eyes in the mature male are separated by a 

 wide, concave space, the primary relations tend to be obscured by secondary modi- 

 fications, such as the forcing upwards and backwards of the roof of the cranium on to 



Fig. <).— Disarticulated frontal buues of a, Scuphthalmus maximus ; b, Hippoglossus hippo- 

 ^lossus ; c, Glyptocephalus cytwglossus ; d, PUuroticctes platessa. [After Traquair.] i., 

 interorbital bar ; i.p., interorbital process ; pfa., pra'frontal articulation. 



the posterior wall, or even over the pectoral arch.' In such forms as Solea and 

 Symphinns the skull is again very specialised, and, as is the case with Bothus, the 

 asymmetry appears to increase with age. 



Traquair's interpretation of the pseudomesial bar as a new formation was in 

 harmony with the assumption that the migration of the eye causes, or is caused by, 

 a twisting of the whole interorbital region of the cranium — a view which seems to have 

 been widely accepted. As far as the skull in general is concerned, it is a fact that 

 the otic and occipital regions have undergone comparatively little change, and in 

 Pscttodes are nearly symmetrical, whereas the orbital region has been greatly modified ; 

 the ethmoid region, apart from the praefrontals, has undergone considerably less 



' Kyle (iqzi, p. /S, pi. V, figs. 12-18). 



