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, etc. 

 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. II. Herrick ('95, p. 143), who states tliat " in Alpheus saulcyi, wlicre tlie 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." 



