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Show 302 DR. A. KEITH ON THE CHIMPANZEES. [Mar. 7, Number of Cusps on the upper Molar Teeth of Gorillas and Chimpanzees. Chimpanzees m.1 4 cusps only. 100 p.c. 100 p.c. Small 5th present. Freq. present. Small6th present. m.2 4 only. 56 pc. 85 p.c. Small 5th. 43 p.c. 15 p.c. Small 6th. 1 p.c. m.3 4 only. 20 p.c. 90 p.c.4 Small 5th. 55 p.c. 10 p.c. Small 6th. 25 p.c. 4th cusp was very much reduced in size in over 50 per cent. Number of Cusps on the lower Molar Teeth of Gorillas and Chimpanzees. Chimpanzees m.i 4 cusps only. 10 p.c. 5th. 90 p.c. 90 p.c. 6th. 10 p.c. m.2 4 only. 5th. 80 p.c. 80 p.c. 6th. 20 p.c. 20 p.c. m.3 4 only. 25 p.c. 5th. 70 p.c. 65 p.c. 6th. 30 p.c. 10 p.c. The differences betAveen the premolar teeth of the Gorilla and Chimpanzees are even more marked than betAveen the molars, and these teeth will probably afford the best clue to the indentification of different races of Chimpanzee. The premolars of the Gorilla are much larger than those of the Chimpanzee and shoAV very little individual variation. The average Length of the Premolars in Gorillas and Chimpanzees. pm.i pin.2 pm.1 pm.2 Gorilla 10 mm. 10 mm. 16 mm. (male). 11mm. 14 mm. (female). 11 mm. Chimpanzee 7"5 mm. 7 mm. 10 mm. (male). 8-5 mm. (male). 9 mm. (female). 7'5 mm. (female). The most characteristic feature of the Gorilla, male and female alike, is the great development of the first lower premolar tooth. To a certain extent this is dependent on the great development of their canines. The upper premolars of the Gorilla are of equal size ; in the Chimpanzee the second premolar is, with occasional exceptions, less than the first and shoAvs in the size of its cusps and the union of its fangs signs of a retrograde development. A feature of the Central-African Chimpanzee is the relative small size of its second premolars, both upper and lower; there is not |