PAKKER: OPTIC CIIIASMA IN TELEOSTS. 233 



sinistral halibuts and starry flounders and by the dextral bastard floun- 

 ders. These two conditions are distinguished not only by differences in 

 the external symmetry of the fishes, but still more so by the optic chias- 

 niata. Thus, in a sinistral species, like Paralichthys californicus, the 

 typical individuals, having their right nerves dorsal, will have their optic 

 chiasmata somewhat uncrossed (Fig. 5), as already explained in dealing 

 with the soles (p. 226), and the atypical individuals, having their right 

 nerves also dorsal, will have their optic crossings emphasized (Fig. 6). 

 Converse conditions occur, of course, in dextral species, such as Pla- 

 tichthys stellatus (Figs. 7 and 8). 



It might at first sight seem that the relations here pointed out are 

 like those already noticed in the Soleidae, but such is not precisely the 

 case. Wlien it is kept in mind that there are two types of cliiasmata 

 and that these may be combined with eyes either on the right or on the 

 left side of the head, it is clear that there must be four possible com- 

 binations. Tlie conditions in any species of sole can be thought of as 

 a combination of one of two types of nerve crossing with eyes always 

 on the same side of the head. The conditions in the three species of 

 Pleuronectidae may be described as a combination of one type of nerve 

 crossing witli the eyes either on the right or the left side of the head. 

 It thus follows that the two combinations in any one species of sole 

 cannot duplicate those in any one species of the Pleuronectidae in which 

 both dextral and sinistral individuals occur. 



IV. The Asymmetry of the Heterosomata. 



The older natiiralists assumed generally that the asymmetry of the 

 flatfishes was simply a question of tire migration of the eye. It is now 

 being recognized that the problem is a much more complex one. Thus 

 Cole and Johnstone (: 01, p. 8) have pointed out that the lack of sym- 

 metry of the mouth is quite independent of that of the eyes, though 

 both are probably adaptations to side swimming. The different colora- 

 tions of the two sides of the body, as well as the unsyra metrical form 

 of the skull, seem to be independent of the migration of the eye. This 

 is proved in pai't by tlie observations of Bumpus ('98, p. 197), who 

 noticed that many specimens of Pseudopleuronectes americanus were 

 marked with dark splotches on their light sides, though otherwise normal, 

 and also by those of Holt ('94) on a solo in which the typical coloration 

 and form of skull were present, though the eye had not migrated. The 



