32 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 Pl'effer'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, wliich fuses with ectoatotic bone-tissue formed 

 on the wings of the ethmoid. 



h. Resume. 



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. Tliere 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 " froutale 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 riglit 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. 



