BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI, 141 
end of the medulla, ventral to it, and just anterior to 
the notochord, lying between the layers of the fibrous 
capsule, is a thick plate of cartilage, connecting the car- 
tilaginous ear capsules. ‘This is the only cartilage found 
in the cranium. 
The cranial cavity is 11 mm. long and 3 mm. deep. The 
width varies, measuring 6 mm. across in the region of the 
olfactory lobes, 5 mm. across at the foramen of the 
trigeminus, and 33 mm. at the foramen of the vagus. 
The lateral and anterior walls of the cranium are pierced 
with numerous openings for the exit and entrance of the 
cranial nerves and the blood vessels that supply the brain. 
Owing probably to the fibrous composition of the cranium, 
the nerve foramina are not definite openings in the wall, but 
rather places where the fibres have been pushed apart, and 
the meshes between them consequently enlarged. ‘Through 
these meshes pass the bundles of fibres composing the nerve 
trunks as threads through a sieve (fig. 10). The inner 
surface of the capsule is lned throughout with a layer of 
endothelial cells. 
The Cranial Membranes.—The brain of Bdellostoma 
has but one enveloping membrane—the dura mater. This is 
composed, as usual, of areolar tissue, and consists of two 
layers—an outer one lying close to the inner surface of the 
cranial capsule, but separated from its lining membrane by 
the extra-dural lymph space, and an inner one following the 
contour of the brain, leaving, however, a small subdural 
space between it and the brain (fig. 7). Both ventrad and 
dorsad of the ’tween-brain, and ventrad of the anterior end 
of the olfactory lobes, one or both of these layers delaminate 
into several finer ones. ‘The space between the layers is 
filled with coagulated lymph, which is infiltrated with connec- 
tive-tissue cells, and shot through with rather fine wavy fibres, 
some running singly, others in small bundles, connecting the 
two layers. These fibres are comparatively few in number 
around the fore part of the brain, but'increase greatly dorsad 
of the cerebellum, and dorsad and ventrad of the medulla. 
