WILLIAMS : MIGRATION OF EYE IN PSEUDOPLEURONECTES. 29 



coming much thinner as it does so, and unites with the upright bar. 

 Thus the foramen for the left nerve (/. s., Fig. 16) has a very thin outer 

 wall, while for the right olfactory nerve (/. dx., Fig. 16) there is no 

 foramen. The olfactory nerves pass under the wings of the ethmoid to 

 the capsules, which are located on the front faces of the wings. 



Since the head of Bothus is less unsymmetrical than that of P. ameri- 

 canus, there is a corresponding difference in the conditions of the supra- 

 orbitals. The right supraorbital (Fig. 16, trb. sii'orb. dx.) is crowded over 

 until it comes to lie directly over the median bar of the ethmoid, which is 

 continued backward into the interorbital septum. There it persists for 

 a distance equal to nearly one-half the diamete rof the eye in all the 

 specimens of Stage IV. (Bothus) which I have sectioned. It should be 

 said that Bothus reaches this turned stage at a much earlier age than 

 does P. americanus. 



The left supraorbital is proportionately of larger diameter than the 

 persisting supraorbital in P. americanus, and it also lies nearer the mesial 

 arch, with which it is often connected. Such a connection sometimes 

 occurs in the winter flounder, the condition of which has been previously 

 described. 



In the older specimens there is no separate supraorbital, but the 

 upper end of the upright mesial cartilage bears a wedge-shaped enlarge- 

 ment on the side toward the left eye (Plate 3, Fig. 16, trb. siCorb. s.). 

 When, in the more posterior sections, the mesial cartilage ends, this 

 enlargement persists, and can be followed iintil it reaches the ear region, 

 thus showing that it is the supraorbital cartilage. The cartilage form- 

 ing the mesial arch is heavier and extends farther back between the eyes 

 than in P. americanus. The result is as if some of the space between the 

 hook and the trabecular cartilage in Stage IV. of P. americanus {ham. 

 eth., Fig. D) were filled out solid, and the whole plate were thickened. 



In the transformation of the cartilaginous skull into the typical 

 condition of the adult teleost, the skull bones, as is well known, may be 

 formed (1) by ossification in the subcutaneous fibrous tissue (paros- 

 tosis), or (2) by ossification between perichondrium and superficial 

 cartilage cells, gradually replacing both by bone (ectostosis). There are 

 no dermostoses, and, as in the case of the salmon (Parker,' 73), I saw no 

 indications of endostosis. Of the bones directly involved in the turn- 

 ing, the frontals originate as parostoses and the pterygo-palatines and 

 pre-frontals as ectostoses. 



