254 EDWARD PHELPS ALLIS, JR. 
gone special development in this fish, and the conditions here 
show, even more positively than in the others considered, that 
part of this bone may be developed in definite relations to the 
membrane separating the myodome into dorsal and ventral 
compartments. It is accordingly certain that this bone is here 
developed, in part, in relation to axial skeletogenous material, 
and is not a simple dermal bone primarily developed in relation 
to the mucous lining membrane of the pharynx, and which . 
sank gradually inward to its actual position. 
DACTYLOPTERUS VOLITANS 
The conditions in Gasterosteus, as above explained, seeming 
to offer an explanation of the somewhat exceptional conditions 
that I described in Dactylopterus in my work on the mail-cheeked 
fishes, I have reexamined my material of that fish. In that 
earlier work I described a transverse ridge on the dorsal sur- 
face of the parasphenoid that was said to project dorsopos- 
teriorly and to form the posterior wall of the myodome. Be- 
cause of the position of this wall, I concluded that the post- 
pituitary portions of the horizontal processes of the prootics had 
been depressed and appressed upon the underlying ventral 
flanges of those bones, and that the latter flanges had under- 
gone marked reduction. I now find that the ventral flanges of 
the prootics have not undergone any particular reduction, and 
that there has been no depression and appression of the hori- 
zontal processes of the prootics, which are represented by a 
well-defined membrane forming the floor of the cavum cere- 
brale cranii.. The anterior end of this membrane passes over 
the dorsal edge of the transverse ridge on the parasphenoid, 
closely adherent to it, and is then continuous with the mem- 
brane that I described as closing the pituitary opening of the 
brain case. Beneath the part of this membrane that repre- 
sents the horizontal processes of the prootics, and between it 
and the parasphenoid, is a space which must represent some 
part of the dorsal compartment of the myodome, this space 
being shut off from the subpituitary portion of the myodome 
by the transverse ridge on the parasphenoid. The hypophy- 
