BRAIN. 153 
aqueduct ending blindly in the mid-brain, in front of which is only the third ven- 
tricle, completely cut off from the rest. The brain of Bdellostoma (fig. 153) 
differs from this in several respects. 
ELASMOBRANCHS (figs. 154, 167) usu- 
ally have the brain somewhat compact, but in 
a fewit is long and slender. The more strik- 
ing features are the slight development of the 
intercerebral fissure, the large hemispheres be- 
ing lateral expansions just in front of the dien- 
cephalon. The optic lobes are large and the 
large cerebellum overlaps both lobes and the 
fossa rhomboidea. The olfactory lobes arise 
from the antero-lateral angle of each hemi- 
sphere; their length varies between wide limits. 
The epithelial roof of the ’twixt-brain is wide 
and bearsa pinealis which often reaches the roof 
of the skull, but the parietal organ is lacking. 
The hypophysis and infundibulum are pro- 
vided with large inferior lobes and a well devel- 
oped saccus vasculosus. The cerebellum hasa 
longitudinal groove and usually one or more 
transverse grooves, dividing the upper surface 
into paired lobes. The medulla differs in the 
sharks and the skates, being very short in the 
latter, much longer in the former. In both the 
corpora restiformia are large folds on either 
side of the cerebellum, in front of and lateral _. 
to the fossa rhomboidea. 
In most elasmobranchs the ventricular sys- 
tem is well developed, but in some the paired 
and third ventricles are not well separated, 
while in the Myliobatide there is no cavity in 
the cerebrum. There is a large epiccele ex- 
tending upward from the aqueduct into the 
optic lobes and a similar cavity usually enters 
the cerebellum. ; 
TELEOSTOMES.—There is a wide range Fic. 154.—Brain of Heptanchus, 
of form in the brain of ganoids and teleosts. It after Gegenbaur. bo, bulbus olfac- 
: F 3 torius; c, cerebrum; cb, cerebellum; 
is usually small in proportion to the size of the gm eminentia teretes; i, infundibu- 
animal and is noticeable for the small size of lum; m, mesencephalon; oo, olfactory 
the telencephalon and the usually non-nervous eles peg er peed 
character of the pallium, which in the teleosts is nial nerves. 
purely epithelial. Consequently the cerebrum 
consists largely of the corpora striata and the intercerebral fissure is slightly de- 
veloped. The paired ventricles are small, but they extend into the olfactory lobes. 
The ’twixt-brain, at a lower level than the rest, has a large infundibulum, saccus 
