198 GORILLAS AND CHIMPANZEES 



the chimpanzee, and remains so until he approaches 

 the adult state ; but as he approaches this period, 

 the ridge above the eyes becomes more prominent, 

 and at the same time a sharp, bony ridge begins to 

 develop along the temples, and continues around the 

 back of the head on that part of the skull called the 

 occiput. At this point it is intersected by another 

 ridge at right angles to it. This is called the sagittal 

 ridge, and runs along the top of the head towards 

 the face ; but on the forehead it flattens nearly to 

 the level of the skull, and divides into two very low 

 ridges, which turn off to a point above the eyes and 

 merges into that ridge. These appear to be a con- 

 tinuous part of the skull, and are not joined to it by 

 sutures. The mesial crest in very old specimens 

 rises to the height of nearly two inches above the 

 surface of the skull, and imparts to it a fierce and 

 savage aspect ; but in the living animal the crests 

 are not seen, as the depressions between them are 

 filled with large muscles, which make the head look 

 very much larger than it would otherwise. These 

 crests affect only the exterior of the skull, and do not 

 appear to alter the form or size of the brain cavity, 

 which is larger in proportion than that of the chim- 

 panzee. These crests are peculiar to the male gorilla, 

 and the female skull shows no trace of them. 



There is at least one case in which this crest has 

 failed to develop in the male. By reference to the 

 series of skulls found in the cuts given herewith. 

 No. 6 is that of an adult male, which I know to be 

 such, as I dissected him and prepared the skeleton 



