226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
two types of chiasmata. The American soles may, therefore, be said to 
be dimorphic in the same sense that symmetrical teleosts are. 
The only representative of the European soles that was studied was 
the common sole, Solea solea (Linn.), or, as it is often called, S. vulgaris 
Quens. All the specimens at hand were dextral. As the Table shows, 
about half had the right nerve dorsal and half the left one dorsal. 
Cunningham (’90, p. 68) states that in this species the left nerve is 
dorsal, but he makes no mention of the number of specimens examined. 
Doubtless his information was based on the inspection of too few 
individuals. 
Of the tongue fishes, which are typically sinistral, observations were 
made on two species, but only in Symphurus plagiusa was the material 
sufficient to yield significant results. Here, as in the American and 
the European soles, both types of crossing were observed, but specimens 
with the left nerve dorsal were much more numerous than those with 
the right nerve dorsal. 
One may conclude from these facts that the species of Soleidae, both 
dextral and sinistral, are characterized, like the symmetrical teleosts, by 
dimorphism in the structure of their optic chiasmata. 
The dimorphism of the Soleidae, since it is accompanied by asymmetry, 
gives rise to rather unusual conditions in the optic nerves, and these con- 
ditions are characteristic for each of the two types of nerve crossing. 
Thus, in a dextral species the individuals with the left nerve (that is, 
the nerve connected with the migrating eye) dorsal have in a measure 
begun to uncross the optic nerves, since the migration of the left eye 
tends to draw the nerve connected with it into a course more nearly 
parallel with the right nerve (cf. Fig. 8); whereas individuals with the 
left nerve ventral have emphasized the crossing of the nerves by having 
the left nerve drawn around the right one by the migration of the left 
eye. Thus, though the Soleidae are like symmetrical teleosts in hav- 
ing two types of optic nerve crossings, their chiasmata are more or 
less pronounced, according as the nerve connected with the migrating eye 
is ventral or dorsal. 
The Pleuronectidae, or flounders, are divisible into some six sub- 
families, three of which are abundantly represented in American waters ; 
these are the Hippoglossinae or halibuts, of which some species are 
dextral and some sinistral, the Pleuronectinae, or flounders proper, which 
with very few exceptions are dextral, and the Psettinae, or turbots, 
which are as a rule sinistral. I have had the opportunity of examining 
in all twenty-eight species of Pleuronectidae. Of these, three were 
