220 Proceedings of the Royal Physical Society. 
I may add that the result of my own examination of the 
structure of the pleuronect head is in entire harmony with 
the above view of the nature and origin of the asymmetry of 
these animals. But M. Steenstrup has only a few months 
ago* revived the theory of Eosenthal, that the upper eye 
has passed through the head to the place it now occupies, 
and that this " migration " of one of the eyes has had a 
much more important share in bringing about the ocular 
transposition, than any slight twisting that may have taken 
place. According to his views, the eye at an early age 
must have quitted its primitive position, and directing itself 
upwards and towards the anterior, pierced the vault of the 
cranium constituted above the eye by the frontal bone, and 
formed for itself a new orbit, whether on the internal region 
of the frontal bone of the same side, or between the two 
frontals." In support of this theory, he refers to the struc- 
ture of several very interesting young pleuronectidae about 
an inch long, brought from various localities in the Atlantic, 
and deposited in the Museum of Copenhagen. In one of 
these specimens, which seem for the most part to belong to the 
group of " Plagusise," the transposition seems never to have 
taken place at all, the eyes being disposed one on each side, 
that on one side being, however, slightly above the level, of 
the other. But the specimen which most fully seems to 
justify M. Steenstrup's views, is one where the animal seems 
to have three eyes, the eye of one side projecting through a 
little fissure above that of the other side, which side thus 
becomes binocular. 
This appearance is so very striking, that we need not 
wonder that M. Steenstrup remarks, " Can we imagine a more 
striking demonstration of the passage of the eye across the 
head than an eye arrested in that position ?" Another speci- 
* J. Japetus Sm. Steenstrup ; — " Om Skjaevheden hos Flynderne, og navnlig 
om Vandringen af det ovre Oie fra Blindsiden till Oiesiden tvers igjennend 
Hovedet." Kiobenhavn, 1864. Saerskilt Aftryk af Oversigt over d. K. D. 
Vid. Selsk. Forhandl. i. Nov. 1863. 
" Observations sur le Developpement des Pleuronectes," par M. Steenstrup. 
(Annales des Sciences Naturelles. Paris, Nov. 1864.) 
The former of these two papers, being written in the Danish language. 1 
have not yet read. 
