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 (J. 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, trd. sw’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. sw’orb. s.). 
When, in the more posterior sections, the mesial cartilage ends, this 
enlargement persists, and can be followed until 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. 
