226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



two types of chiasmata. The Aiuericau soles may, tlierefore, be said to 

 be diinorpliic in the same sense that symmetrical toleosts 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 tliat 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 witli 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 hift 

 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 



