DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDE. 265 
embryo*; but he seems to me to be the first to appreciate the morphological value of 
such phenomena. 
But very recently Steenstrup has 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 
tmnsposition than any slight twisting that may have taken place T. According to his 
views, “The eye, at an early stage, must have quitted its primitive position, and, 
directing itself upwards and towards the interior, pierced the vault of the cranium con- 
stituted 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 appearances presented by several very in- 
teresting young Pleuronectidæ, each about an inch long, brought from various localities 
inthe Atlantie, and deposited in the Museum at Copenhagen, and directs especial at- 
tention to one in which one eye seems to be arrested in the process of piercing the head. 
In addition, M. Steenstrup remarks that the osteology of the head of the adult flatfish 
confirms his view of the process of ocular transposition in the embryo. Like Rosenthal, 
he compares the head of a flatfish to that of a Cyclopean malformation, and affirms that 
the position in which we find the upper eye is not homologous with that oceupied by the 
lower, nor with the orbit of any other fish or vertebrate animal in general. 
I ean only say that the results of my own investigations do not agree with those state- 
ments of the above-quoted distinguished naturalist. What the views are which I have 
adopted will appear in the following paper; meanwhile I will only remark that the ap- 
Pearances presented by the cranium of the adult flatfish seem to me to be at complete 
variance with any theory that the two eyes of these animals occupy morphologically 
different positions from each other. In this communication I have named the bones 
according to the nomenclature given in Professor Owen's ‘ Lectures on Comparative 
Anatomy ;’ but, in doing so, I do not wish to be considered as committing myself to any 
| a general morphological ideas which may be associated with that nomenclature. 
vm must, however, have names ; and so long as our investigation does not trench on 
general question of the homologies of the vertebrate skeleton, one system of names, 
Provided it be widely known, may be used as advantageously as any other. 
I. On the Cranium of the Pleuronectide. 
a studying the cranium of the flatfishes, we must take into account t 
mbrane, which form morphologically as integral a part of the skeleto 
he cartilage and 
n as the bones 
*(6 i. 
ases of similar monstrosities or arrestments of development had been previously recorded, by Donovan as 
« 
"init Cyclops,” by Schleep as ** Rhombus maximus duplex," and by Yarrell. 5 
fra Blin oum 3m. Steenstrup “Om Skjaevheden hos Flynderne, 0g navnlig "s 
. Vid n til Oiesiden tvers igjennend Hovedet.”  Kjóbenhavn, 1864. Saerskut 
S I Forhandl. i. Nov. 1863. 
i E sur le Développement des Pleuronectes," par M. Steenstrup (Àn 
andringen af det óvre Oie 
Aftryk af Oversigt over d. 
nales des Sciences Naturelles, Paris, 
Nov, 
I have not yet read. 
The fo 
rm LI 
er of these two papers, being written in the Danish language; 2-3 
pe ee 
