222 bulletin: museum of comparative zoology. 



the chiasma, but he farther remarked that this relation is not constant, 

 and that individual difi'erences occur. Owen ('66, p. 300) observed that 

 the nerves cross each other without interchange of fibres, and that some- 

 times the nerve of the right eye is dorsal, as in the hake, and sometimes 

 that "of the left, as in the halibut. He added in a note that both con- 

 ditions had been seen in different individuals of the cod. Gegenbaur 

 ('98, p. 796), in his recent comparative anatomy, reiterates the chief 

 statement made by Stannius ; namely, that the right nerve is usually 

 dorsal, but he cites no examples supporting this opinion. C. J. Ilerrick 

 ('99, p. 394), in his work on Menidia, remarks that in this fish the 

 left nerve is dorsal, as " is typical for teleostomes," and in this state- 

 ment I understand him to mean the nerve connected with the left eye, 

 an interpretation already put on this passage by Cole and Johnstone 

 (:01, p. 116). Finally Greeff (: GO, p. 25), in the new edition of the 

 Graefe-Saemisch Handbuch der Augenheilkunde, reaffirms the statement 

 originally made by Stannius that the right nerve is dorsal. Thus there 

 is a difference of opinion as to which nerve usually is dorsal, — a con- 

 dition of affairs that can be cleared up only by reinvestigation. 



Much of the material upon which the following studies were made, 

 was either from the collections of the Museum of Comparative Zoology 

 or from those of the United States Fish Commission. To the officers of 

 both these institutions I express my grateful thanks. The materials 

 obtained from each of the two sources are indicated by foot-notes in con- 

 nection with the Tables ; material not otherwise designated was obtained 

 by myself. 



II. Positions of the Nerves in the Chiasmata of Symmetrical 



Teleosts. 



To ascertain whether the right nerves or the left nerves are more 

 usually dorsal at the chiasmata of symmetrical teleosts, I examined a 

 hundred specimens each of ten common species. The results of this 

 examination are given in Table I., in which the columns opposite the 

 name of the fish show the number of instances of right nerves dorsal 

 and of left nerves dorsal in a total of one Jbundred cases. These two 

 conditions, as Owen ('66, p. 300) long ago observed, are well shown in 

 the cod (Figs. 1 and 2). 



This table shows that in six of the ten fishes examined (Fundulus, 

 Rhombus, Stenotomus, Tautoga, Prionotus, and Melanogrammus) the 

 left nerve was dorsal about as frequently as the right, the greatest dif- 



