302 



DE. A. KEITH ON THE CHIlirAXZEES. 



[Mar. 7, 



Number of Cusps on the xipper Molar Teeth of Gorillas and 

 Chimpanzees. 



* 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. 



The differences between the premolar teeth of the Gorilla and 

 Chimpanzees are even more marked than between the molars, and 

 these teeth will probably afford the best clue to the indentitication 

 of different races of Chimpanzee. The premolars of the Gorilla 

 are much larger than those of the Chimpanzee and show very little 

 individual variation. 



The average Length of the Premolars in Gorillas and Chimpanzees. 



pm . ■ 



pm.- 

 Gorilla 10 mm. 10 mm. 



Chimpanzee 7o mm. 7 mm. 



pm.' 



16 mm. (male). 

 14 mm. (female). 

 10 mm. (male). 

 9 mm. (female). 



pm.- 



11 mm. 



11 mm. 



8'5 mm. (male). 



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 shows 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 



