224 ' BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
each of the ten species examined, these species may be said to be 
dimorphic in this respect, and one might naturally ask whether this 
dimorphism is correlated with other characters such as sex, race, ete. 
To the question, Is the dimorphism of the chiasma correlated with sex ? 
a conclusive answer can be given, for two of the ten species examined. 
In Fundulus of the 51 specimens with left nerves dorsal 29 were females 
and 22 males, and of the 49 with right nerves dorsal, 29 were females, 
and 20 males. Of the 43 specimens of Tautogolabrus with the left 
nerves dorsal 26 were females, and 17 males; and of the 57 with right 
nerves dorsal, 26 were females, and 31 were males, These figures show 
clearly that there is no close correspondence between the crossing of the 
optic nerves and sex. 
Whether or not the two types of nerve crossing represent racial differ- 
ences,! cannot at present be decided. In Fundulus, Menidia, Tautogo- 
labrus, Tautoga, and Prionotus the whole material came in each instance 
from a very restricted area, presumably from a single colony, and yet 
both conditions were abundantly present. But evidence of this kind 
is obviously very inconclusive, and a satisfactory answer to this question 
can probably be obtained only by experiments in breeding. 
It thus appears that symmetrical teleosts are from the standpoint of 
their optic chiasmata dimorphic, and that their optic nerves cross with- 
out either nerves being preponderantly dorsal, a condition of approxi- 
mate equality not previously recognized. 
III. Positions of the Nerves in the Chiasmata of the 
Heterosomata. 
From the symmetrical teleosts one naturally turns to the flatfishes as 
a group whose lack of symmetry, particularly in the positions of the eyes, 
invites study. In the older classifications these fishes constituted one 
family, the Pleuronectidae ; in more recent taxonomic works, such as 
that by Jordan and Evermann (96-00), the group is raised to a sub- 
order, Heterosomata, and divided into two families, the Pleuronectidae, 
or flounders, and the Soleidae, or soles. This separation agrees well 
with the facts to be given in the subsequent part of this paper and will, 
1 For a good instance of this kind among the Crustacea, we are indebted to 
F. H. Herrick (’95, p. 148), who states that “in Alpheus saulcyi, where the large 
crushing chela can be recognized even before the animal is hatched, the members 
of a brood are either right-handed or left-handed; that is, have the crushing claw 
on the same side of the body.” 
