Evolution of the Order Ileterosomata. 485 



o£ the eyed side, displaced outwards and downwards ; the 

 frontal of the blind side is broad and may send forward a 

 process to share in the formation of the interorbital bar, but 

 the main part of this bone in the orbital region is on the wrong 

 side of its eye, although its relations are normal in other 

 respects. The last-named fact leads one to suspect the 

 correctness of Traquair's interpretation of this part of the 

 frontal of the blind side, which he regards as a new process 

 sent forwards to join the prefrontal in order to form a bar or 

 bridge supposed to be requisite for the stability of the 

 cranium. 



Traquair's * elaborate descriptions and figures of the crania 

 of several flat-fishes are most valuable, and his interpretation 

 is in harmony with the often repeated statement that the 

 migration of the eye causes or is caused by a twisting of the 

 whole orbital region of the skull, and has been generally 

 accepted ; but recent embryological work does not, in my 

 opinion, bear out this view. As is well known, flat-fish larvae 

 have the eyes on opposite sides and swim vertically, and at an 

 early age one eye migrates round the top of the head to 

 the other side, which is thenceforth uppermost. 



Williams f lias studied the migration of the eye in Pleura- 

 nectes americanus ; in the cartilaginous cranium there are two 

 supraorbital bars, precursors of the frontal bones, connecting 

 the lateral ethmoids with the otic capsules ; preparatory to 

 the metamorphosis there is a rapid resorption of the part of 

 the supraorbital bar which lies in the path of the migrating 

 eye, so that this bar becomes reduced to a forwardly directed 

 process of the otic capsule and a backwardly directed one of 

 the lateral ethmoid. The eye migrates between these two 

 projections, and so approaches the supraorbital bar of the 

 future eyed side ; the eyes then move to their final position, 

 causing a torsion of this supra-orbital bar, which also affects 

 the ethmoid region ; after the shifting is complete, ossifi- 

 cation takes place. 



From this account it seems that it is wrong to say that 

 the two eyes are on one side as the result of the twisting of 

 the orbital region of the skull, for the first step is a 

 migration of one eye into the territory of its frontal bone, 

 causing resorption of cartilage in the larva, and in the adult 

 producing the effect that the orbital part of its frontal ossifies 

 round it or even entirely outside it. The displacement of the 

 frontal of the lower eye has enlarged the area of that of the 



* Trans. Linn. Soc. xxv. 1865, pp. 263-296, pis. xxix.-xxxii. 

 t Bull. Mas. Comp. Zool. xl. pp. 1-57, pis. i.-v. (1902). 



