247 
quite certainly corresponds, as in Lepidosteus, to the inner wall of the 
acustico-trigemino-facialis recess in certain specimens of Chlamydo- 
selachus, that recess of the latter fish being included in the chamber 
of Amia. This being so, the middle one of the three walls potentially 
present in this region in fishes has disappeared in Amia, as in Lepi- 
dosteus, and, in addition, the pituitary fossa and the trigemino-facialis 
chamber have fused to form a single chamber, the so-called eyemuscle 
canal (myodome) of the fish. Associated with this fusion of these 
two chambers, that part of the anterior portion of the prootic bridge 
that lies, on either side, lateral to the foramen for the nervus abducens 
does not undergo either chondrification or ossification, and the middle 
portion of the bridge is left projecting into the cranial cavity of the 
prepared skull, between the abducens nerves of opposite sides of the 
head, exactly as the dorsum sellae does in man. The interclinoid 
and petrosphenoidal ligaments of human anatomy are then simply 
parts of the inner membranous wall of the myodome of Amia, and the 
membrane that gives origin to these igaments in man may undergo, 
in other vertebrates, more or less complete ossification. The internal 
carotid artery enters the cranial cavity, in man, lateral to the base 
of the dorsum sellae, between the sphenoid and temporal bones, 
while in Amia it perforates the preclinoid bolster (tuberculum sellae). 
But this apparently radical difference in the course of this artery is 
probably due simply to the less complete development of the hind 
ends of the trabeculae in man, the carotid artery, in consequence, 
entering the cranial cavity, as it does for the same reason in teleosts, 
between the probable homologues of the lateral wing of the para- 
sphenoid and the prootic bone. That semicircular notch on the 
posterior surface of the tuberculum sellae of man that forms the 
anterior end of the carotid groove must then represent a remnant 
of the carotid canal of Amia. The internal jugular vein of Amia, as 
it traverses the trigemino-facialis chamber, lies, as it does in teleosts, 
at first ventro-internal to the trigemino-lateralis portion of the tri- 
gemino-facialis ganglionic complex. It then traverses that ganglion, and 
so acquires, in its posterior portion, a position dorso-external to the 
nervus facialis (Allis, 1897), lying always lateral to the projecting 
anterior portion of the prootic bridge. The anterior section of this 
jugular vein thus corresponds in position to the sinus cavernosus of 
man, which sinus lies lateral to the body of the sphenoid and hence 
lateral also to the dorsum sellae. 
