238 EDWARD PHELPS ALLIS, JR. 
the hind ends of the recti interni, the recti superiores arising 
from the lateral walls of this cavity, near its anterior end. The 
recti externi lie dorsal to the parasphenoid, between it and the 
membrane which everywhere forms the roof of the myodome. 
Still further forward, the bony roof of the median cavity in 
the parasphenoid is gradually replaced by a horizontal myo- 
domic membrane which separates the myodome into two com- 
partments, a dorsal one lodging the recti externi and a ventral 
one lodging the recti interni and superiores, the ventral compart- 
ment forming a semicircular depression in the floor of the entire 
myodomie cavity (fig. 21). The hind end of the ventral com- 
partment is thus completely enclosed in the parasphenoid, and 
it seems absolutely certain that that part of the bone forming 
the roof of this compartment is simply an ossification of the 
horizontal myodomic membrane. 
Proceeding anteriorly, the region of the ascending processes of 
the parasphenoid is soon reached, these processes rising to the 
level of the membranous roof of the myodome and suturating 
with the ventromesial edges of the prootics (fig. 20). The 
myodome is here semicircular in transverse section, its side wall 
and floor being formed wholly by the parasphenoid and its roof 
by membrane that separates it from the cavum cerebrale cranii. 
A median vertical membrane here descends from the membranous 
roof of the myodome, and in connection with it the recti infe- 
riores have their origins. 
The course and the main branches of the internal carotid 
artery are as in Scomber, except that the artery separates into 
its anterior and posterior divisions while still within the canal 
in the median vertical myodomic membrane, and that the 
anterior division then immediately enters the tissues forming 
the floor of the cavum cerebrale cranii, thus not actually enter- 
ing the latter cavity. 
HIPPOCAMPUS GUTTULATUS 
In a 20-mm. specimen of this fish the myodome begins pos- 
teriorly beneath a part of the basis cranii that is of cartilage 
lined, on either side, with perichondrial bone. The parasphe- 
noid lies at a certain distance ventral to this part of the basis 
