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 
differ 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 teleosts 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 nerve was dorsal 514 times, the 
left 486 times. 
3. The two types of chiasmata are not correlated with sex. 
4, Inthe Soleidae the chiasmata are also dimorphic, as in symmet- 
rical teleosts. 
5. In the Pleuronectidae the chiasmata are monomorphic for each 
species ; in dextral species the left nerve is dorsal, in sinistral species 
the right nerve 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 1V.), 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 Pleuronectidae, but degenerate 
descendants of primitive flatfishes, from which the Pleuronectidae have 
probably been derived. 
9. The monomorphic 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. 
