MYODOME AND TRIGEMINO-FACIALIS CHAMBER 301 
and I concluded that there must be some error in the descrip- 
tions, for I did not question the identification of the veins. Be- 
cause of the position of this vein, I concluded that the cavum 
epiptericum was a trigemino-facialis chamber. ‘This is, how- 
ever, wrong, for the so-called sinus cavernosus is, in reality, the 
homologue of the pituitary vein of fishes, and not of the vena 
jugularis. This vein of Echidna must then traverse a myo- 
domic cavity, as it does in the Mammalia ditremata, and there 
must be a membrane separating it from the cavum epiptericum, 
that membrane being a part of the membrana limitans of Terry’s 
(17) descriptions of the cat and forming the roof of a myodomic 
cavity which is the sinus cavernosus properly so-called. The 
pituitary vein then traverses this cavity, as it does in man, and 
that part of the so-called sinus which Gaupp says turns later- 
ally and falls into the sinus transversus, is the vena encephalica 
of fishes, this latter vein falling into the vena capitis lateralis 
(vena jugularis of fishes) after and not before, it issues from the 
cavum epiptericum. The vena capitis lateralis has here, as 
in the Mammalia ditremata, lost its primitive continuity with 
the vena capitis media; the persisting portions of these veins 
both lie external to the cranial wall; and the cavum epiptericum 
is a trigemino-facialis recess. The conditions in this animal 
are then stricty similar to those in the Mammalia ditremata 
except that a taenia clino-orbitalis has been formed, compar- 
able to, but somewhat different from, the cartilage ‘c’ of Voit’s 
descriptions of the rabbit. 
In the adult Echidna it would seem, from Gaupp’s descrip- 
tions, as if certain of the bones forming the lateral wall of the 
cranium were developed in the lateral wall of the caxum epip- 
tericum (trigeminus recess), and certain of them in the lateral 
wall of the pars jugularis of a trigemino-facialis chamber, for 
certain of the bones are said (I. c., p. 650) to be ossifications of 
the membrana spheno-obturatoria, which is said to lie external 
to the ala temporalis. The taenia clino-orbitalis is said by 
Gaupp (l. c., p. 647) to have fused, in the adult, with the lateral 
edge of the sella turcica along the full length of the sella, the 
fissura pseudo-optica thus being greatly reduced in size; the 
JOURNAL OF MORPHOLOGY, VOL. 32, NO. 2 
