Brain and Cranial Nerves of Acanthias vulgaris. 189 
because in every case it remains in close proximity te the muscles derived 
from the wall of the second head-cavity of the embryo. 
With regard to the brains themselves, the relative proportions of their 
various parts can be seen by comparing the figures. That of Pristiwrus, 
however, requires particular notice in connection with the chief sense organs. 
The sense organs of the skin are well developed: the otocyst is extra- 
ordinarily small; the eyes are of considerable size; and the olfactory organs 
are enormous, the pair taking up the whole width of the head where they 
are situated and projecting as well (Fig. 11). When we examine their 
respective brain-lobes we find that the Somatic Sensory Column is better 
developed than in the other forms (Fig. 15), and the Lobi Linie Lateralis are 
particularly large and conspicuous. Sagittal section through the Medulla 
brings out another point. In addition to the usual broad band connecting 
the two inner walls of the Restiform Bodies in the median plane just behind 
the Cerebellum, there is a smaller one (Fig. 15). That is, the Lobi Liniz 
Lateralis as they go forward fuse together in the middle line before diverging 
to form the outer walls of the Restiform Bodies. The Auditory nerves are 
small, and the Optic Lobes are in proportion to the size of their sense organ, 
but the Olfactory Lobes are not enlarged at all above the normal and are 
therefore extraordinarily small relative to the organ they supply. ‘This lack 
of correlation between the sizes of the sense organ and of its brain lobe 
reminds us of the eyes and Corpora Quadrigemina of Mammalia which show 
the same peculiarity (e.g. Mycticebus, Nyctipithecus, Tarsius). 
As a whole the Hemisphere region of these two forms is simpler than 
that of Scyllium,; there are but slight indications of paired swellings. In 
Galeorhinus there are dorsal and antero-ventral swellings, the latter slightly 
lobed in front and the former a little behind, while in Pristiwrus there is a 
very slight indentation anteriorly and a mid-ventral groove, but there is 
nothing even so well marked as the pair of prominences on the dorsal surface 
of Acanthias. 
As seems to be general among the sharks, the Fore-brain in both these 
fish does not nearly fill its part of the cranial cavity. In Galeorhinus the 
unoccupied space, which is of very considerable size (Fig. 6), is filled with 
arachnoid fluid and fibres as is the case in Hchinorhinus (Jackson and Clarke), 
and in Lepidosiren (Graham Kerr). 
But the part of these brains which most attracts our attention is the 
Cerebellum. In Pristiwrus it is rather wide and flattened, and has two 
sulci—a median longitudinal one along the posterior two-thirds, and a deep 
transverse one in front of the longitudinal one at about a quarter of the 
whole length of the Cerebellum from its anterior end. The depth of this 
