258 ON THE STRUCTURE OF THE SKULL. 



length to be, in a well-developed European skull, 266 ; in a 

 Negro, 236 ; in an adult female Chimpanzee, 180 ; in an adult 

 male Gorilla, 170; in a Baboon, 144; in a Lemur, 119; in a 

 Dog, 87 ; in a Beaver, 70 ; in a Thylacinus, 60 ; in an Opossum, 

 93; in Echidna, 100. 



2. In the lower Mammalia the olfactory, tentorial, and occi- 

 pital angles nearly approach right angles ; or, 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 Mammalia (Fig. 100) the cranio-facial angle 

 is so open as to reach 150° or more, but, in the higher Mammalia, 

 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 hemispheres 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 low r er Mammalia the olfactory fossa is altogether 

 in front of the cerebral cavity, and the cerebellar fossa is alto- 

 gether behind it, the three being separated by marked con- 

 strictions (Fig. 100). 



Fig. 100. 



Pirtx 



Fig. 100.— Longitudinal and vertical section of the skull of Tlvjltcinus cynoceph'ih**. 



In the higher Mammals, 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 



