[Illustration: FIG. 28.--Longitudinal and vertical sections of the skulls of a Beaver (_Castor Canadensis_), a Lemur (_L. Catta_), 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. 29) as 236 to 100. In the Constantinople skull (Fig. 29) 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 _d^1d^2_, &c., 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.] The diagrams in Figure 29 are reduced from very carefully made diagrams of sections of four skulls, two round and orthognathous, two long and prognathous, taken longitudinally and vertically, through the middle. The sectional diagrams have then been superimposed, in such a manner, that the basal axes of the skulls coincide by their anterior ends, and in their direction. The deviations of the rest of the contours (which represent the interior of the skulls only) show the differences of the skulls from one another, when these axes are regarded as relatively fixed lines. The dark contours are those of an Australian and of a Negro skull: the light contours are those of a Tartar skull, in the Museum of the Royal College of Surgeons; and of a well developed round skull from a cemetery in Constantinople, of uncertain race, in my own possession. It appears, at once, from these views, that the prognathous skulls, so far as their jaws are concerned, do really differ from the orthognathous in much the same way as, though to a far less degree than, the skulls of the lower mammals differ from those of Man. Furthermore, the plane of the occipital foramen (_b c_) forms a somewhat smaller angle with the axis in these particular prognathous skulls than in the orthognathous; and the like may be slightly true of the perforated plate of the ethmoid--though this point is not so clear. But it is singular to remark that, in another respect, the prognathous skulls are less ape-like than the orthognathous, the cerebral cavity projecting decidedly more beyond the anterior end of the axis in the prognathous, than in the orthognathous, skulls.