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Show 258 ON THE STRUCTURE OF THE SKULL. length to be, in a well-developed European skull,. 266 ; in a Ne-gr·o , 236 ,. in an adult female Chirnpan. zee, 180; In an a.d ult male Gorilla, 170; in a Baboon, 144; 1n a Le~ur, 119 ; In a Dog, 87; in a Beaver, 70; in a Thylacinus, GO; In an Opossum, 93 ; in Echidna, 100. 2. In the lower Ma1nrnalia the olfactory, tentorial, and occi-pital angles nearly approach right a~gles ; ~r, in other words, the corresponding planes are nearly vertical, while they become more and more obtuse in the higher Mammals, until, in Man, these planes are nearly horizontal, in the ordinary position of the skull. 3. In the lower Mamraalia (Fjg. 100) the cranio-facial angle is so open as to reach 150° or more, but, in the higher Marnrnalia, it becomes smaller and smaller, until, in Man, it may be as little as 90°. 4. In many of the lower Mammalia, a sudden narrowing of the front part of the cranial cavity indicates the boundary between the chamber which lodges the cerebral hen1ispheres and that which contains the olfactory lobes of the brain (Fig. 100), and the latter cavity forms a large and distinct olfactory fossa. In the higher Mammals this cavity becomes absolutely and relatively smaller, until in Man it is so shallow and insignificant as to be hardly noticeable. 5. In many lower Mammalia the olfactory fossa is altogether in front of the cerebral cavity, and the cerebellar fossa is altogether behind it, the three being separated by marked constrictions (Fig. 100). Fig. 100. Fig. 100. - Longitudinal an1l ,·ertical section of the skull of Thylucin11s cynocephrtllls. In the higher Matnmals, on the other hand, the excessive development of the cerebral hemispheres causes the cerebral chamber to overlap the olfactory fossa in front and the cerebellar THE SKULL OF MAMMALIA. 259 fossa b~hind; so. that these como to be placed respectiv ly under, Instead of In front of and behind, the cerebral chamber. Thus it may be ~aid, that in passing from the lower to the higher forms of Mammalian skull, we find the cavity for the cerebrum enlarging in proportion to the basi-cranial axis, and thrusting the olfactory plano downwards and forwards, the tentorial and occipital planes downwards and backwards, in such a manner that these may be said to rotate on the ends of the basi-cranial axis; at the same time, the basi-facial line rotates on the basi· cranial line, being more and more bent downwards and backwards. It must be clearly understood that I by no means intend to suggest that all Mammalian skulls can be arranged in a series, the lower members of which shall be distinguished from the higher by always exhibiting smaller olfactory and occipital angles, larger cranio-facial angles, less proportional cerebral lengths, &c. On the contrary, the various angles and measurements show a considerable range of irrelative variation; as, for example, in the Cetacea, a relatively large cerebral length is associated with small occipital and olfa,ctory angles, and a very large craniofacial angle; in the Edentata and Monotremata a somewhat large olfactory angle is associated with a small tentorial and occipital angle; and in the Opossun1 and Echidna the cerebral length is anomalously great. All that can be said is, that the crania of the higher orders of Mammals, as a whole, are distinguished from those of the lower orders by the characters I have rnentionod. ~flhe skull of Echidna (Figs. 101 and 102) may be taken as an example of the "aberrant" monotren1e type of skull. It is composed of a pyriform cranium proper, and a produced, beak-like maxillary portion. The lower jaw is remarkable for its length and slenderness, and the very s1nall vertical height of its rami (Fig. 101). The basi-occipital (B.O.) is very wide, and so much depressed as to be quite a thin lamella of bone; it contributes, to a small extent, to each occipital condyle, which, like the ex-occipital bone itself, is very large. The ex-occipitals are connected above by n. wide supra-occipital, which extends so far upon the roof of s 2 |