MYODOME AND TRIGEMINO-FACIALIS CHAMBER 217 
gain additional height because of the gradual widening of the 
dorsal portion of the myodomic cavity, cross-sections through 
which change gradually from oval to pear-shaped and then to 
triangular. The roof of, the myodome thus becomes flat (fig. 6,) 
instead of being arched (fig. 7). The cartilage forming the top 
of the arched roof is continued forward as the median portion of 
the flat roof, and is enclosed between plates of perichondrial 
bone which do not meet in the median line and which form the 
distal (mesial) portions of the horizontal processes of the pro- 
otics. The lateral portions of these processes and the dorsal 
portions of the side walls of the myodome are now each formed 
by two plates of bone, doubtless of perichondrial origin but 
without enclosed cartilage, this bone replacing the cartilage 
of the preceding sections and forming the dorsolateral corners 
of the myodomie cavity. The horizontal portion of each of 
these angles of bone forms the basal (lateral) portion of the 
horizontal process of the prootic of its side, and arises from 
the base of the lateral wall of the cavum cerebrale cranii. The 
vertical portion of the angle of bone forms a wall between the 
dorsal portion of the myodome and the ventral portion of what 
is, in the prepared skull of the adult, a large bay on the exter- 
nal surface of the cranium. This bay forms that part of the 
large auditory fenestra of Ridewood’s (’04) descriptions that 
lies anterior to the so-called vertical lamina of the prootic, and 
its floor is formed, in my embryo as in the adult, by a laterally 
projecting, horizontal shelf of the prootic. This bay of this 
fish corresponds to the facialis part of the trigemino-facialis 
chamber of my description of Scomber and the mail-cheeked 
fishes, and occupies a position, relative to the cranial: walls, 
similar to that of the recessus sacculi, the floor of the bay being 
an anterior continuation of that of the recessus. The truncus 
hyomandibularis facialis enters this bay through a foramen 
in its mesial, cranial wall, and runs outward above its floor. 
The vena jugularis, traced from behind forward, enters the bay 
over the posterior edge of its floor, accompanied by a sym- 
pathetic nerve, a communicating branch from the nervus glosso- 
pharyngeus to the nervus facialis, and the arteria carotis ex- 
terna, this artery lying ventromesial to the other structures. 
