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 Rosenthal, 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 pleuronectidse 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 <l Plagusiee/' 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 Srn. Steenstrup ; — " Om Skjaevheden hos Flynderne, og navnlig 

 om Vandringen af det ovre 6'ie fra Blindsiden till Oiesiden tvers igjennend 

 Hovedet." Kidbenhavn, 1864. Saerskilt Aftryk af Oversigt over d. K. D. 

 Vid. Selsk. Forhandl. i. Nov. 1863. 



" Observations snr 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. I 

 have not yet read. 



