o2 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
layer of epidermis over the corneas of both eyes in the oldest fishes which 
I have sectioned, as indeed one would expect ; so that Pfeffer’s statement — 
apparently would have been more accurate if he had said “ Lederhaut ” 
instead of ‘‘ Korperhaut.” 
Unless the conditions in the species described by Pfeffer are totally 
different from those found in P. americanus and Bothus, Pfeffer has not 
distinguished between the cartilaginous supraorbital bar, which may be 
in direct connection with the cartilaginous wings of the ethmoid, and 
the dermal frontal bone, which fuses with ectostotic bone-tissue formed 
on the wings of the ethmoid. 
h. Résumé. 
The twisting which takes place in the ethmoid region of the skull of 
Pleuronectidae can best be explained by reference to the three mutually 
perpendicular axes of the head of the symmetrical young. There are 
two important torsions of about 90 degrees each. The most evident 
change (incidentally described by those who have discussed the migra- 
tion of the eye) is that twisting of the ethmoids which can be rep- 
resented by the revolution of the horizontal transverse axis until it 
approximately coincides with the original dorso-ventral axis. 
The second change (limited to the upper part of the ethmoid mass) 
results in carrying the dorsal end of the dorso-ventral axis forward, 
so that it coincides with the longitudinal axis of the head. This change 
is probably due to growth along the anterior face of the ethmoids and 
resorption of the posterior dorsal margin, which is pressed upon by the 
eyes, or to a gradual displacement of the cartilage, due to the pressure 
referred to, without absorption. 
In Pseudopleuronectes there is a further complication due to a slight 
retrocession of the parts on the eyeless side, amounting to about 30 de- 
grees. This obliquity does not exist in Bothus. 
The changes which have been described in the head of the flounder 
all take place in the cartilaginous skull, ossification occurring only after 
the shifting is complete. Therefore I cannot accept Pfeffer’s view that 
a portion of the ‘“‘frontale principale” lying in the path of the migrating 
eye is resorbed. The history of the two supraorbital cartilages links to- 
gether to some extent the cartilaginous and bony conditions. The 
supraorbital cartilage bar next the migrating eye (the left in P. ameri- 
canus, the right in Bothus) degenerates in its middle region, and the 
eye is carried through the gap thus made by the unequal growth of the 
facial cartilages of the two sides. 
