556 
the tentorial surface of the cerebellum may be taken as a measure 
of the amount of the posterior part of the cerebrum by which it is 
covered. Its lateral ventricles possess no traces of posterior horns. 
I have already mentioned that both Tiedemann and Cuvier have 
noted that the seal possesses more largely developed cerebral hemi- 
spheres than the Carnivora generally ; and Cuvier places, along with 
the seal, the otter. I have, as yet, had no opportunity of examining 
the brains of these animals, but the accompanying casts of the in- 
terior of their cranial cavities will give some conception of the re- 
lations of the cerebellum and cerebrum.* 
In the otter, the cerebrum not merely covered the tentorial aspect 
of the cerebellum, but even projected beyond it in a very striking 
manner. Thus, when the brain was looked at from above, no part 
of the cerebellum was exposed. From the cast, it would appear as 
if the occipital surface of the cerebellum looked almost directly back- 
wards. The cerebral hemispheres possessed considerable width 
posteriorly. In the seal, nearly the same relations prevailed as in 
the otter ; the posterior projection of the cerebral hemispheres was 
more strongly marked laterally than in the middle line. This was 
due partly to the ossification of the tentorium and falx cerebri, and 
partly to the posterior cerebral fossse not passing quite so far back 
in the middle line as they did somewhat further outwards. 
An inspection of the interior of the cavity of a cranium, in my 
possession, of a walrus ( Trichecus ), an animal closely allied to the 
seal, led me to suppose that, if a cast of the cavity were taken, re- 
lations of a similar nature would be met with. I, accordingly, made 
such a cast, and found that the cerebral hemispheres projected back- 
wards beyond those of the cerebellum; this projection, as in the seal, 
and from the same cause, being more strongly marked laterally than 
in the middle line. The occipital surface of the cerebellum was 
almost flat, and directed backwards, with but a slight upward in- 
clination. Both the seal and the walrus may be considered to 
possess brains of large size, so that the cerebellar hemispheres were 
concealed by the cerebral lobes when the brain was looked at from 
above. In them as well as in the otter the cerebellum was inferior. 
* Most anatomists, I think, will admit that a very correct general conception of 
these relations may be obtained in those cases where it is difficult to procure the 
brains themselves, by making casts of the cranial cavity. The accuracy of this 
method is ensured, even more absolutely, when the tentorium is ossified. 
