232 bulletin: museum of comparative zoology. 



Of the halibut, liippoglossus hippoglossus, thirteen specimens were 

 examined, twelve dcxtrnl and one sinistral, and in all the left optic 

 nerve was dorsal, thus confirming the statement of Owen ('66, p. 300) 

 for this species. Of the bastard halibut, Paralichthys californicus, 

 twenty-six were examined, eleven sinistral and fifteen dextral, and in 

 all the right nerve was dorsal. Of the starry flounder, Platichthys 

 stellatus, one hundred were examined, fifty sinistral and fifty dextral, 

 and in all the left nerve was dorsal. It therefore appears that each 

 of these three species has a monomorphic chiasma irrespective of the 

 fact that it may be composed in part of sinistral and in part of dextral 

 individuals, and, therefore, the conclusion is that, at least in these 

 species, the manner of the crossing of the optic nerves is independent of 

 the type of migration shown by the eye. 



The three species mentioned seem at first sight to be exceptions to 

 what has been said of the Pleuronectidae in general, but such is not 

 wholly true. Each species, as in the other Pleuronectidae examined, 

 has a monomorphic chiasma, and the nerve that is dorsal in each instance 

 is the one that would reasonably be expected to be. Thus, in the halibut 

 the species is essentially dextral, for sinistral individuals are extremely 

 rare,^ and in conformity with this the left nerve is always dorsal. The 

 bastard flounder belongs to a genus all other American members of which 

 are sinistral ; it is therefore natural to find that in this species, though 

 it contains both dextral and sinistral individuals, the rule for a sinistral 

 form holds, the right nerve being always dorsal. The starry flounder 

 is a member of the Pleuronectinae, a subfamily in which this species is 

 almost the only American exception to complete dextrality, and as 

 usual the rule for dextral species prevails, all left nerves being dorsal. 

 These species, therefore, conform perfectly to the rule for other Pleu- 

 ronectidae that prescribes a monomorphic chiasma, and though in them 

 the dorsal nerve is not always connected with the migrating eye, it is 

 always connected with that eye which in the greater number or nearest 

 of kin is the one to migrate. Thus these species are not so exceptional 

 as they at first appear. 



Of the two conditions presented by each -of the three species men- 

 tioned one may be said to be typical and tbe other atypical. The 

 typical condition is represented by the dextral halibuts and stai-ry floun- 

 ders and by the sinistral bastard halibuts ; tlie atypical condition by the 



1 The sinistral halibut examined by me was the only individual obtained dur- 

 iiict the winter of 1900-01 by one of tlic larjicst halibut estabhshmcnts in Boston. 

 It was certainly a single individual u\ many thousands. 



