MYODOME AND TRIGEMINO-FACIALIS CHAMBER 283 
pronounced posteclinoid and preclinoid walls. Bing says the 
hypophysis lies in the posterior portion of this fossa, the an- 
terior portion being filled with arachnoidal tissue (Arachnoideal- 
maschen). The postclinoid wall is evidently formed by growth 
of the epichordal and hypochordal bands of parachordal carti- 
lage said by Greil to enclose the tip of the notochord in embryos. 
The preclinoid wall had not begun to be developed in the oldest 
embryos described by Greil. No foramina leading into the 
pituitary fossa are shown or described by any of these three 
authors. 
I find, in an old and somewhat dissected skull of this fish, 
a perforation of the cartilage of the basis cranii at the bottom 
of the posterior portion of the pituitary fossa, and it is closed 
by tough membrane. <A small canal in the cartilage leads from 
either orbit to the edge of this membrane and must certainly 
have transmitted a vein which either traversed the membrane 
or passed dorsal to it, in order to reach and drain the hy- 
pophysis. The space traversed by this vein, wherever it may 
be, is a dorsal myodomic cavity. The internal carotid artery 
of either side passes internai to the parasphenoid, is there 
joined by the efferent pseudobranchial artery (mandibular aortic 
arch of Greil’s descriptions), and then becomes embedded in the 
cartilage of the basis cranii and covered externally by membrane. 
The arteries of opposite sides are connected by a cross-commis- 
sural vessel which lies posterior to the median perforation in 
the floor of the pituitary fossa, the canal traversed by this cross- 
commissure representing part of a ventral myodomic cavity. 
Anterior to this cross-commissure each artery runs forward 
ventral to the pituitary vein, sends forward the arteria palatina, 
and then certainly enters the pituitary fossa through a foramen 
that I find lying anterolateral to the median perforation in the 
floor of the fossa, but, as my skull had been cleaned and the 
arteries removed, I cannot definitely establish this. If it tra- 
verse this foramen, as seems certain, it must enter and traverse 
that anterior portion of the pituitary fossa which Bing says is 
filled with arachnoidal tissue, this part of the fossa then repre- 
senting the internal carotid canals of Amia, fused with each 
