188 Proceedings of the Royal Physical Society. 
The last part of the brain to be described is the Hemisphere region, and 
it is of considerable interest because it differs strikingly from what obtains 
in Scyllium. The posterior part of the dorsal wall is thin, non-nervous and 
vascular, and from it folds hang down into the cavity forming a choroid 
plexus (Fig. 5). In front of this the walls are thick, and the anterior and 
ventral ones are expanded on either side of the middle line, so that when 
viewed from above the region exhibits a deep median notch in front, and — 
from below a median longitudinal groove (Figs. 2 and 3). In spite of these 
expansions, however, this portion of the brain does not nearly fill the cavity 
of the cranium, but ends considerably behind the anterior wall (Fig. 1) and 
does not reach the membranous roof above. , | 
On the dorsal surface there are a pair of swellings as in Scyllawm, but they 
are not so conspicuous. On the other hand, the Olfactory tracts, which are 
given off laterally from the paired anterior lobes, are much longer and more 
slender than is the case in the majority of the sharks. This is correlated 
with the shape of the snout, which is long and pointed. | 
The cavity of the Hemisphere region is not complicated above the normal. 
It is single behind, and this unpaired portion opens in front into the first 
two ventricles of the brain, from which the canals of the Olfactory tracts 
pass directly. 
From the centre of the anterior wall, 7.¢. at the bottom of the median 
notch, a pair of quite small ribbon-like nerves arise which follow the contour 
of the anterior wall of the brain and Olfactory tracts, and go to the Olfactory 
capsules. Owing to their being ganglionated and their fibres myelinated, 
both distinguishing features from ordinary Olfactory nerves, the morphology 
of these Pre-olfactory Nerves (Nervi Terminales) is of considerable interest. 
(The nerves are not shown in the figures.) 
II.—THE BRAINS AND CRANIAL NERVES OF GALEORHINUS 
AND PRISTIURUS. 
Turning now to Galeorhinus and Pristiwrus, what chiefly strikes us with 
regard to the Cranial Nerves is their close agreement with one another and 
with those of Seyllvwm. . 
The nerve showing the greatest variation is the Profundus. This, of 
course, is absent altogether in Scylliwm as a separate tract. In Galeorhinus 
it runs over the origin of the Rectus Superior but then curves down and 
passes under the Rectus Internus and Obliquus Superior muscles (Fig. 6). 
In Pristwwrus it lies ventral to all these three muscles. So in each case— 
Acanthias, Galeorhinus, or Pristiurus—it takes a different course. That this 
is so, however, hardly affects the discussion as to. its morphological standing, 
