238 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Among arthropods the presence of enlarged chelae on one or other side, 

 as already mentioned, may involve discontinuity. The same is true of 

 the sexual asymmetry of the Cyprinodonts as worked out by Garman 

 ('95), and it is probable that the condition in the human being known 

 as situs transversus viscerum is of like nature. Thus many other ani- 

 mals show in the reversal of asymmetrical conditions evidence of dis- 

 continuous variation not unlike that of the flatfishes ; but the flatfishes 

 di(fer from many of these in the relatively high degree of stability that 

 their asymmetry possesses, — a condition in part explainable, in my 

 opinion, as the result of the association of a special form of asymmetry 

 with certain advantageous internal conditions, like a particular type 

 of optic nerve crossing. 



V. Summary. 



1 . In each of ten species of symmetrical tcleosts the optic chiasmata 

 were dimorphic, in that in some instances the right optic nerve was 

 dorsal, in others the left. 



2. In a thousand cases the right uerve was dorsal 514 times, the 

 left 486 times. 



3. The two types of chiasmata are not correlated with sex. 



4. In the Soleidae the chiasmata are also dimorphic, as in symmet- 

 rical tcleosts, 



5. In the Pleuronectidae the chiasmata are monomorphic for each 

 species ; in dextral species the left nerve is dorsal, in sinistral species 

 the right uerve is dorsal. 



6. All species of Pleuronectidae that turn in only one direction have 

 their dorsal nerves connected with their migrating eyes. In all species 

 that have both dextral and sinistral individuals (Table IV.), the dor- 

 sal nerve is connected with that eye which in the greatest number or 

 in the nearest of kin migrates. 



7. The unmetamorphosed young of the Pleuronectidae are not sym- 

 metrical in the same sense that symmetrical teleosts are, for they have 

 monomorphic chiasmata. 



8. The Soleidae are not degraded Plcnronectidae, but degenerate 

 descendants of primitive flatfishes, from which- the Pleuronectidae have 

 probably been derived. 



9. The monomorpliic condition of the optic chiasma of the Pleu- 

 ronectidae can be explained only on the assumption of natural selection. 



10. The flatfishes afford striking examples of discontinuous variation. 



