240 
minum separated from the nervus trigeminus by a bar of tough 
connective tissue. 
If these four selachians be now compared it will be seen that 
there is, in each of them, excepting only Mustelus, a definitely devel- 
oped acustico-trigemino-facialis recess, and that this recess may be 
closed toward the cranial cavity by a wall of tough connective tissue. 
The lateral wall of the recess forms the lateral wall of the chondro- 
cranium in Chlamydoselachus and Heptanchus, but in the latter fish 
there is still another wall, of membrane, outside this cartilaginous 
wall; and in the space between this outer membranous wall and the 
chondrocranium lie the external carotid artery and the ramus palatinus 
facialis. In Mustelus this outer membranous wall is partly chondrified 
so as to enclose the external carotid, and the internal jugular vein 
is here enclosed in a partly formed membranous canal. In Acanthias 
the outer membranous wall has become still further chondrified, and 
the internal jugular vein, the external carotid artery and the palatinus 
facialis are all enclosed in separate cartilaginous canals, the internal 
jugular and external carotid canals opening anteriorly into the trige- 
mino-pituitary fossa. If these two latter canals and the canals for the 
nervus trigeminus, nervus facialis and the basal portion of the ramus 
palatinus facialis were to fuse, it is evident that a chamber would be 
formed in the side wall of the chondrocranium the floor of which 
would be perforated by the ramus palatinus, and that this chamber 
would correspond strictly to the trigemino-facialis chamber of Scomber 
and Scorpaena; and this seems certainly to have been the way in 
which the latter chamber has arisen. If the proximal portion of the 
canal for the palatinus facialis were included in this fusion, but not 
the canal for the nervus facialis, the palatinus would enter the chamber 
by a foramen which would apparently correspond strictly to the 
hiatus Falloppu of the human skull; and other variations would arise 
according to the extent and nature of the fusions of the several canals 
involved. The chamber would still be wholly separate and distinct 
from the pituitary canal, but the orbital opening of the chamber, 
no longer a simple foramen trigemini, would be connected with the 
pituitary foramen by a groove which would lodge the internal jugular 
vein and be the homologue of the posterior portion of the jugular groove 
of my descriptions of the Mail-Cheeked fishes. The beginnings of a 
myodome would exist in the pituitary canal, into which, in Acanthias, 
the rectus externus has already found entry. 
