BRAIN AND GRANIAL NERVES OF BDELLOSTOMA DOMBRYI. 159 
cerebellum ; this condition is shown in fig. 14. Cephalad of 
this strands are seen running from the acusticum athwart 
the arms of the Y to the sides of the medulla. ‘These are 
composed of fibres of the acustico-lateral nerve acusticus a, 
and of the ear nerves—acusticusc and acusticus d. 
The Fourth Ventricle——Leaving the external anatomy 
of the medulla of Bdellostoma, and turning to a study of 
sections through it, its most striking feature is found to be 
the smallness of the fourth ventricle, and the entire absence, 
except for about 11 mu behind the cerebellum, of the exceed- 
ingly thin roof found over the fourth ventricle of other 
vertebrates. ‘lhe fourth ventricle of Bdellostoma is com- 
posed of three distinct parts, an anterior and a posterior 
dilatation and a connecting canal. ‘The anterior dilatation is 
1:07 mm. long, ‘036 mm. wide, and ‘59 mm. deep. It les in 
the cephalic end of the medulla, and is almost entirely 
covered by the cerebellum. ‘The thin part of the roof les 
over the hind end of this dilatation. ‘he iter leaves the 
fourth ventricle at its cephalo-dorsal angle, and immediately 
ventral to the iter isa small finger-like diverticulum (figs. 
5, 6). The hind end of the dilatation gives off several small 
diverticula dorsal to the connecting canal. ‘he number of 
these varies in different specimens, one brain having two, 
and two others three. Sometimes one of these diverticula 
may divide into two. ‘I'he connecting canal leaves the 
anterior dilatation at its caudo-dorsal angle, and runs straight 
caudad to the canal of the spinal cord. It is ‘065 mm. in 
diameter. ‘lhe posterior dilatation is the most curious part 
of the fourth ventricle. It begims ‘57 mm. cephalad of the 
commissura infima Halleri, and ends ‘21 mm. caudad 
of it, having a total length of -78 mm. Before it is 
reached the connecting canal has become narrower, so 
that it is about half as wide as it is deep. ‘I'he posterior 
dilatation is semicircular in cross section, ‘168 mm. wide 
and ‘073 mm. deep, and is placed dorsal to the canal, 
which opens into it. Canal and dilatation together look, in 
cross section, like a longitudinal section of a toadstool. As 
