MYODOME AND TRIGEMINO-FACIALIS CHAMBER 211 
ally, on either side, with the related alisphenoid, and anteriorly 
with the orbitosphenoid. It is perforated by a single median 
foramen which transmits the two optic nerves, this apparently 
agreeing with Ridewood’s description of this bone, for his 
statement that it ‘‘forms the superior edge of the optic foramen’”’ 
must mean that the two optic nerves traverse it through a single 
opening. 
As already stated, the groove on the ventral surface of the 
basioccipital of my 5l-mm. specimen lodges the anterior por- 
tion of the median dorsal aorta. When, proceeding anteriorly, 
the aorta begins to widen, preparatory to separating into a 
lateral dorsal aorta on either side, it recedes from the groove 
and is replaced by the hind ends of the musculi recti externi; 
these muscles soon occupying the entire groove, the aorta lying 
ventral to them and outside the groove. The lateral edges of 
the groove give insertion to the tunica externa of the air- 
bladder, the tissues of the tunica forming, in the posterior, but 
not the anterior portion of the groove, an arched bridge beneath 
the aorta and so enclosing it in a canal; this being as described 
by Bridge (99) in. Notopterus. The notochord, enclosed in 
the basioccipital, lies directly above the bottom of the groove, 
separated from it by only a thin layer of bone of perichordal 
origin. 
In sections through the extreme hind end of the basioccipital 
(fig. 12) the aortal groove is shallow and les directly beneath 
the notochord, between blocks of cartilage which are unques- 
tionably the homologues of the lower arches, or basiventrals, 
of current descriptions of the vertebrae, but which I shall refer to 
as the ventrolateral vertebral processes. On each of these proc- 
esses there are two slight ridges: a ventromesial one, clothed 
with perichondrial bone that represents a part of the hind end 
of the basioccipital, and a dorsolateral one, not clothed with bone, 
which gives attachment to a ligament running outward in an 
intermuscular septum and doubtless representing either a dorsal 
or a ventral rib. In sections slightly farther forward (fig. 11) 
the ventrolateral cartilaginous processes have entirely disap- 
peared, but are represented by parts of the basioccipital which 
