THE SKULLS OF MAMMALIA. 259 



fossa behind ; so that these come to be placed respectively 

 under, instead of in front of and behind, the cerebral chamber. 



Thus it may be said, that in passing from the lower to the higher 

 forms of Mammalian skull, Ave find the cavity for the cerebrum 

 enlarging in proportion to the basi-cranial axis, and thrusting 

 the olfactory plane 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 olfactory angles, and a very large cranio- 

 facial angle; in the Edentata and Monotremata a somewhat- 

 large olfactory angle is associated with a small tentorial and 

 occipital angle ; and in the Opossum 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 dis- 

 tinguished from those of the lower orders by the characters 

 I have mentioned. 



The skull of Echidna (Figs. 101 and 102) may be taken as an 

 example of the " aberrant " monotreme type of skull. It is com- 

 posed 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 small 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 a wide supra-occipital, which extends so far upon the roof of 



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