230 EDWARD PHELPS ALLIS, JR. 
face of the tissue. The horizontal membrane is strongly devel- 
oped here, and extends across the median line between the 
ventral ends of the persisting portions of the cartilaginous side 
walls, thus lying at a certain distance dorsal to the parasphenoid. 
Proceeding posteriorly from here, in the sections, the recti interni 
disappear, leaving a space between the horizontal membrane 
and the parasphenoid. The latter bone then shortly disappears, 
and the myodomiec cavity is then closed ventrally by the hori- 
zontal membrane only, this condition possibly persisting to the 
hind end of the myodome, but my sections are here imperfect 
and I cannot definitely determine this. 
The conditions in Scomber would thus arise from those in 
Hyodon if the ventral compartment of the myodome of the lat- 
ter fish were extended posteriorly nearly to the hind end of the 
aortal groove, and the continuous myodomic-aortal cavity so 
formed where closed ventrally, to that point, by the para- 
sphenoid. 
The internal carotid artery of either side enters, as in Hyodon, 
the ventral compartment of the myodome, runs forward in it 
into the prespinal portion of the myodome, and there turns up- 
ward in the median vertical myodomic membrane, anastomos ng, 
while in the membrane, with its fellow of the opposite side. 
Leaving its fellow, it separates, as in Hyodon, into anterior and 
posterior divisions both of which run upward, posterior to the 
basisphenoid, and enter the cavum cerebrale cranii. The pos- 
terior division sends branches to the hypophysis and then sepa- 
rates into anterior and posterior branches, the anterior branch 
running forward along the floor of the cavum cerebrale cranii, 
sending a branch outward with the nervus opticus, and then 
joining the anterior division of the artery, this anterior pro- 
longation of this branch of the artery not being found in Hyodon. 
The anterior divison of the artery runs forward, dorsal to the 
basisphenoid, and, anterior to that bone, enters the thick, dense 
tissues forming the floor of the cavum cerebrale cranii and the 
dorsal end of the interorbital septum, its course and distribu- 
tion from there onward being as in Hyodon. The fact that 
this anterior division of the artery runs forward dorsal to the 
