[Illustration: FIG. 128.--_Platophrys lunatus_ (Linnæus), the Wide-eyed Flounder. Family _Pleuronectidæ_. Cuba. (From nature by Mrs. H. C. Nash.)] 6. The greater part of this rotation (three-fourths of it in _P. americanus_) is a rapid process, taking not more than three days. 7. The anterior ethmoidal region is not so strongly influenced by the twisting as the ocular region. [Illustration: FIG. 129.--Young Flounder, just hatched, with symmetrical eyes. (After S. R. Williams.)] 8. The location of the olfactory nerves (in the adult) shows that the morphological midline follows the interorbital septum. 9. The cartilage mass lying in the front part of the orbit of the adult eye is a separate anterior structure in the larva. 10. With unimportant differences, the process of metamorphosis in the sinistral fish is parallel to that in the dextral fish. 11. The original location of the eye is indicated in the adult by the direction first taken, as they leave the brain, by those cranial nerves having to do with the transposed eye. 12. The only well-marked asymmetry in the adult brain is due to the much larger size of the olfactory nerve and lobe of the ocular side. 13. There is a perfect chiasma. 14. The optic nerve of the migrating eye is always anterior to that of the other eye. [Illustration: FIG. 130.--Larval Flounder, _Pseudopleuronectes americanus_. (After S. R. Williams.)] [Illustration: FIG. 131.--Larval Flounder, _Pseudopleuronectes americanus_. (After S. R. Williams.)] "The why of the peculiar metamorphosis of the _Pleuronectidæ_ is an unsolved problem. The presence or absence of a swim-bladder can have nothing to do with the change of habit of the young flatfish, for _P. americanus_ must lose its air-bladder before metamorphosis begins, since sections showed no evidence of it, whereas in _Lophopsetta maculata_, 'the windowpane flounder,' the air-sac can often be seen by the naked eye up to the time when the fish assumes the adult coloration, and long after it has assumed the adult form. "Cunningham has suggested that the weight of the fish acting upon the lower eye after the turning would press it toward the upper side out of the way. But in all probability the planktonic larva rests on the sea-bottom little if at all before metamorphosing. Those taken by Mr. Williams into the laboratory showed in resting no preference for either side until the eye was near the midline. "The fact that the change in all fishes is repeated during the development of each individual fish has been used to support the proposition that the flatfishes as a family are a comparatively recent product. They are, on the other hand, comparatively ancient. According to Zittel flatfishes of species referable to genera living at present, _Rhombus_ (_Bothus_) and _Solea_, are found in the Eocene deposits. These two genera are notable in that _Bothus_ is one of the least and _Solea_ the most unsymmetrical of the _Pleuronectidæ_.