WILLIAMS : MIGKATION OF EYE IN PSEUIjOPLEURONECTES. 11 



3. Homologies of the Anterior Bones of the Skull. 



The changes in the cartilaginous facial skeleton will be more easily set 

 before the reader, if the homologies of the bones of the face as explained 

 by the more recent writers be first made clear. 



The papers of PfefFer ('86, '94), which deal with the cartilaginous 

 skeleton, are also reviewed here. 



Traquair ('65) has given a careful account of the adult skulls of 

 flounders of both dextral and sinistral types. The greatest changes, as 

 compared with a symmetrical fish, the cod, he finds in the facial region ; 

 the brain case remaining nearly symmetrical, except with regard to the 

 position of the ridges and wings on the bodies of the bones for the at- 

 tachment of muscles. 



The adult skulls of (1) the halibut, (2) the pole flounder, and (3) the 

 plaice (Platessa vulgaris) form a series, in which he shows that there is 

 a progressive modification, especially of the frontal bones. In the hali- 

 but, though the main part of the frontal of the " eyeless " side is back 

 of the migrating eye, a thin curved process from it extends between the 

 two eyes and with the corresponding interocular process of the frontal 

 of the ocular side (to which it is closely applied) forms a part of the 

 orbit of the migrating eye. In the case of the pole flounder this process 

 from the frontal of the eyeless side is reduced to an exceedingly thin 

 curved strip. Finally, in the common flounder even this thin strip has 

 entirely disappeared, so that the frontal of the eyeless side is now joined 

 with the front of the head exclusively by means of the great externa] 

 connection, since called by German writers the "Brtlcke."^ 



Steenstrup ('63), according to Thomson ('65), considered the " Brtlcke " 

 the principal frontal of the eyeless side. 



Thomson himself thought that it represented the prefrontal of the 

 eyeless side, and that the partition between the eyes was the frontal of 

 the ocular side. 



Malm ('68) at first held the " Brticke " to be infraorbital, but later 

 adopted Steenstrup's view. 



Reichert ('74), disregarding the beliefs of previous authors, decided 

 that the frontal formed two infraorbital processes, which then fused with 

 the latent " Brticke " to form the orbital ring. The parts between the 

 eyes he thought were normal. 



* This is a new and peculiar bridge or bar fpseudomesial) of bone wbich has no 

 (single) equivalent in the crania of synunetrical fishes. 



