On the Development of the Pleuronectidce. 221 
men described by him has one eye on each side, but above 
the left eye is a little slit where the other should appear. 
In addition, M. Steenstrup remarks that the osteology of 
the head of the adult flat fish confirms his view of the process 
of ocular transposition in the embryo. Like Kosenthal he 
compares the head of a flat fish to that of a cyclopean mal- 
formation ; and afiirms that the position in which we find 
the upper eye is not homologous with that occupied by the 
lower, nor with the orbit of any other fish or vertebrate ani- 
mal in general. Here I will in the meanwhile only say that 
my own dissections have led to a very opposite conclusion, 
namely, that the upper eye of a flat fish is homologous in 
position with the lower one — that the bar of bone be- 
tween the eyes is the only representative of the frontal arch 
in the symmetrical osseous fish, but altered in position, be- 
ing pushed over to one side, and that the other bar of bone 
bounding the orbit and upper eye on the inner side is an 
entirely additional formation, developed from the frontal and 
prefrontal of the eyeless side, and not found in the plan of 
the ordinary fish head. 
Although M. Steenstrup's observations are very remark- 
able, and not to be put aside simply because they do not tally 
with our preconceived notions, yet on the other hand I do 
not think that we can at present accept them implicitly as 
representing the normal process of development among all the 
Pleuronectidse, especially seeing that they are in such direct 
opposition to the teachings of the structure of the adult 
pleuronect, both normal and malformed, and to the embryo- 
logical observations, so far as they have gone, of other 
authors. M. Steenstrup's specimens may possibly be malfor- 
mations of a nature essentially similar to those already re- 
ferred to, but in which the dorsal fin ma}^ have actually 
bridged over the upper eye. Or there may be some groups 
of Pleuronectidse, in which the upper eye in the normal 
course of development may become bridged over by the ad- 
vancing dorsal fin, a slit being left through which afterwards 
the upper eye passes to its position on the binocular side of 
the fish. This is, however, mere theory ; meanwhile it is 
greatly to be desired that additional information be collected 
