'occipital angle,' and the angle made by the line 'a d'. with 'a b'. be
termed the 'olfactory angle,' and that made by 'i T'. with 'a b'. the
'tentorial angle,' then all these, in the mammal in question, are nearly
right angles, varying between 80 degrees and 110 degrees. the angle 'e
f b'., or that made by the cranial with the facial axis, and which may
be termed the 'cranio-facial angle,' is extremely obtuse, amounting, in
the case of the Beaver, to at least 150 degrees.

But if a series of sections of mammalian skulls, intermediate between a
Rodent and a Man (Fig. 28), be examined, it will be found that in the
higher crania the basicranial axis becomes shorter relatively to the
cerebral length; that the 'olfactory angle' and 'occipital angle'
become more obtuse; and that the 'cranio-facial angle' becomes more
acute by the bending down, as it were, of the facial axis upon the
cranial axis. At the same time, the roof of the cranium becomes more
and more arched, to allow of the increasing height of the cerebral
hemispheres, which is eminently characteristic of man, as well as of
that backward extension, beyond the cerebellum, which reaches its
maximum in the South America Monkeys. So that, at last, in the human
skull (Fig. 29), the cerebral length is between twice and thrice as
great as the length of the basicranial axis; the olfactory plane is 20
degrees or 30 degrees on the 'under' side of that axis; the occipital
angle, instead of being less than 90 degrees, is as much as 150 degrees
or 160 degrees; the cranio-facial angle may be 90 degrees or less, and
the vertical height of the skull may have a large proportion to its
length.

It will be obvious, from an inspection of the diagrams, that the
basicranial axis is, in the ascending series of Mammalia, a relatively
fixed line, on which the bones of the sides and roof of the cranial