THE EXTERNAL FOEM OF ANTHROPOID APES. 29 



chirapanzee. In this case also we must consider 

 successively the aged and young male, and the aged 

 and young female animals. 



The full-grown chimpanzee is smaller than the 

 adult gorilla. In this species also the male is larger 

 than the female. The chimpanzee is, speaking 

 generally, of a slighter build than the gorilla. 



The head *of the aged male chirapanzee funda- 

 mentally differs from that of the aged male gorilla, 

 since the skull of the former has a depressed crown, 

 and the transverse occipital ridge is only faintly 

 indicated. Since the orbits are also less strongly 

 developed than in the aged male gorilla, and the 

 spinous processes of the cervical vertebrse do not 

 assume the same elevated form which is characteristic 

 of the latter species, the countenance of the chim- 

 panzee is not of a square shape, and there is not space 

 for the strong muscular system arching over the neck 

 like a cowl, which is so characteristic of the gorilla. 

 The head of the chimpanzee displays, both in aged 

 and young specimens, the concave neck which is 

 common among apes, that is to say, a depression 

 between the head and the throat. In an aged male 

 the crown of the head presents a rounded, arched 

 contour, since, as we have already said, the prominent 

 bony processes are wanting. Although the supra- 

 orbital arches are not so excessively prominent as in 

 a gorilla of the same age, they are strongly developed, 

 covered with wrinkled skin, and in this case also 

 there is a species of eyebrow, stiff and bristly, with 

 shorter hairs between. The large, wrinkled lids 

 are furnished with thick eyelashes. The inner 



