PARKER: OPTIC CHIASMA IN TELEOSTS. 238 
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- 
mata. 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 (Tig. 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 m the Soleidae, but such is not precisely the 
case. When it is kept in mind that there are two types of chiasmata 
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. The 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 with 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 naturalists assumed generally that the asymmetry of the 
flatfishes was simply a question of the 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 unsymmetrical form 
of the skull, seem to be independent of the migration of the eye. This 
is proved in part by the 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 sole in which the typical coloration 
and form of skull were present, though the eye had not migrated. The 
