[Illustration: FIG. 29.--Longitudinal and vertical sections of the skulls of a Beaver ('Castor Canadensis'), a Lemur ('L. Catia'), and a Baboon ('Cynocephalus Papio'), 'a b', the basicranial axis; 'b c', the occipital plane; 'i T', the tentorial plane; 'a d', the olfactory plane; 'f e', the basifacial axis; 'c b a', occipital angle; 'T i a', tentorial angle; 'd a b', olfactory angle; 'e f b', cranio-facial angle; 'g h', extreme length of the cavity which lodges the cerebral hemispheres or 'cerebral length.' The length of the basicranial axis as to this length, or, in other words, the proportional length of the line 'g h' to that of 'a b' taken as 100, in the three skulls, is as follows:--Beaver 70 to 100; Lemur 119 to 100; Baboon 144 to 100. In an adult male Gorilla the cerebral length is as 170 to the basicranial axis taken as 100, in the Negro (Fig. 30) as 236 to 100. In the Constantinople skull (Fig. 30) as 266 to 100. The cranial difference between the highest Ape's skull and the lowest Man's is therefore very strikingly brought out by these measurements. In the diagram of the Baboon's skull the dotted lines 'd1 d2', etc., give the angles of the Lemur's and Beaver's skull, as laid down upon the basicranial axis of the Baboon. The line 'a b' has the same length in each diagram.] In such a mammal as a Beaver (Fig. 29), a line ('a b'.) drawn through the bones, termed basioccipital, basisphenoid, and presphenoid, is very long in proportion to the extreme length of the cavity which contains the cerebral hemispheres ('g h'.). The plane of the occipital foramen ('b c'.) forms a slightly acute angle with this 'basicranial axis,' while the plane of the tentorium ('i T'.) is inclined at rather more than 90 degrees to the 'basicranial axis'; and so is the plane of the perforated plate ('a d'.), by which the filaments of the olfactory nerve leave the skull. Again, a line drawn through the axis of the face, between the bones called ethmoid and vomer--the "basifacial axis" ('f e'.) forms an exceedingly obtuse angle, where, when produced, it cuts the 'basicranial axis.' If the angle made by the line 'b c'. with 'a b'., be called the '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.