1899.] DR. A. KEITH ON THE CHIMPANZEES. 311 



measures on an average 25 mm. high and 2-5 mm. broad ; iu the 

 female Gorilla 29 mm. high and 31 mm. broad; 31 mm. high and 

 38 mm. broad in the adult male Gorilla. The orbits vary according 

 to age, sex, and the individual, but they measure, almost without 

 exception, most in the transverse diameter in the Chimpanzee, 

 in the vertical in the Gorilla, a feature dependent on teeth 

 development. 



Certain features in the foramina for the exit of nerves in the 

 facial part of the skull separate the Gorilla and Chimpanzee. The 

 infra-orbital foramen in the Gorilla is divided into two or more 

 compartments by a vertical bar, that iu the Chimpanzee by a hori- 

 zontal bar. This difference depends on the fact that the infra- 

 orbital nerve in the Gorilla sinks down within the maxilla from the 

 mai'gin of the orbit ; in the Chimpanzee it passes horizontally 

 inwards from the maxillar^^-malar sutui-e. The foramina for the 

 nasal nerves are always to be seen in the nasal bones ot the Gorilla, 

 never in those of the Chimpanzee ; in the Chimpanzee these 

 foramina occur in, or at the side of, the upper part of the premaxilla. 

 Malar foramina occur only occasionally in the Gorilla ; they are 

 always present in the Chimpanzee. The inferior palatine foramen 

 of the Chimpanzee is divided by a process of bone — a division not 

 seen in the Gorilla. 



The supra-orbital ridges in Johanna project from the frontal 

 bone to a depth of 22 mm., and are separated by a glabellar notch. 

 This notch is very seldom seen iu the female and never in the male 

 Gorilla. It rarely occurs in the male Chimpanzee and is variable in 

 the female, but does not appear to be coulined to any particular 

 race. The supra-orbital ridges keep on growing through life : in 

 5 young adult Chimpanzees their average depth was 14 mm., in 



4 old adults IS mm. ; at corresponding periods of the same sex of 

 the Gorilla they measure^O and 2-5 mm. In this feature Johanna 

 resembles the Gorilla. These ridges begin to form before the 

 milk-dentition is completed, and the part they play in the animal 

 economy is to strengthen the facial portion of the skull to give a 

 firm dental support. 



The skull of the Chimpanzee is the more hrachycephalic. The 

 average length of 10 skulls of Gorillas, excluding from the measure- 

 ments the prominence due to the frontal air-siuusss and the 

 external occipital protuberance, was 118 mm., the corresponding 

 measurement iu 10 skulls of Chimpanzees 103'6 ; the breadth of 

 skull, from one parietal eminence to another, was in the first 94 mm., 

 in the second 89 mm. The breadth of the skull in Gorillas is 

 80 per cent, of the length, in Chimpanzees 86 per cent. But 

 the measurements overlap, and many of the measurements of the 

 female Gorilla correspond to those of the female Chimpanzee. 



The temporal ridges in .Johanna are about 25 mm. apart on the 

 crown of the head, a Chimpanzee character. In only 1 out of 



5 adult female Gorillas had these ridges not fused into a median 

 crest, and in that particular case the cranial capacity was un- 

 commonly great. In it the temporal ridges were 20 mm. apart. 



21* 



