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Show 1899. DR. A. KEITH ON THE CHIMPANZEES. 301 The palate of the Central-African Chimpanzee most resembles that of the Gorilla. The average area for 3 males amounted to 4350 c.c, rather less than the ordinary Chimpanzee ; the breadth is only 71 per cent, of the length-a very low amount. The skulls of Anthropopithecus calvus and A. kooloo-kamba are too few to draw inferences from, but in both the breadth index is over 80 per cent. The difference in form and size of the teeth of Gorillas and Chimpanzees is very emphatically marked. The cusps of the molars of the Gorilla are extremely prominent, almost prismatic, Avith the enamel deposited in a sharp crystalline manner, with only round the bases of the cusps evidence of the crenated folds of enamel which form a pronounced character in the teeth of Chimpanzees. The cusps of the Chimpanzee are bluntly conical and not nearly so prominent as in the Gorilla. The crenation of the enamel is perhaps the most diagnostic feature of the great Anthropoids. Cusps resembling those of the Gorilla occur in the teeth of the Siamang and some South-American monkeys (Brachyteles and Lagothrix), and represent the molar cusp at its most robust development. The cusps of the Central-African Chimpanzee most resemble those of the Gorilla, but never approach them in degree of development. The molar teeth of the Gorilla, as may be seen from the accompanying measurements, are very much larger than those of the Chimpanzee:- m.1 m.2 4 Length of molar teeth, ] stated in mm., an average I 146 ( X 14) 15'2 of both sexes of Gorilla... J Do. Chimpanzee 10 (x 10-5) 10-2 One may say, almost with certainty, that any upper molar tooth over 12 m m . in length is that of a Gorilla, and under 12 is that of a Chimpanzee. The molar teeth of the female Gorilla are almost as large as those of the male : the molars of the female Chimpanzee are smaller than those of the male and show more marks of retrogression : Avhile the third molar of the Gorilla, especially the lower, is as fully developed as the other two teeth, the corresponding tooth in the Chimpanzee, as in Man, and as in the Orang, shows distinct retrograde changes. The table on p. 302, the result of the examination of 22 Gorilla and 26 Chimpanzee skulls, shows the retrograde development of the cusps in the Chimpanzee, especially in the third molar tooth. The observations show that in point of size, in development of cusps, and in arrangement of enamel the teeth of the Gorilla far exceed those of the Chimpanzee, and, unlike former points of difference, the distinction between the molars of the females is as well drawn as between the molars of the males. In every point the teeth of the Central-African Chimpanzee make the nearest approach to the Gorilla; the molars of the Bald Chimpanzee have probably undergone the most retrograde change. m.3 m.1 m.2 m.3 14-1 15 16 17 8 11-5 12 11 |