8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



beyond the extreme posterior edge of the bone. This region of 

 attachment forms a thin, sharply-defined ledge beneath the pit in 

 which the other tongue-muscles are inserted. While the lower border 

 is essentially alike in the two genera the pit is deeper and narrower 

 in Pan than in Pongo and its upper border is usually well-defined 

 by an abrupt convexity in the posterior profile of the symphysis; the 

 hinder margin of this convexity lying at level of canine or anterior 

 premolar. In both genera the region of temporal muscle-insertion is 

 characterized by the presence of a distinct and narrow ridge curving 

 upward from behind the alveoli and extending to or above the middle 

 of the coronoid process. While they thus agree in certain characters 

 the two genera differ from each other in the form of the symphysis, 

 which, like the entire horizontal ramus, is deeper in Pongo than in 

 Pan. The base of the articular process in Pan is strengthened by 

 a conspicuous ridge extending obliquely downward on the inner side 

 of the mandible. In Pongo this ridge is barely indicated. Below the 

 ridge in Pan a distinct groove extends upward and backward from 

 the dental foramen; this is scarcely visible in Pongo. Turning to 

 Gorilla it is seen that the digastric muscle pushes conspicuously 

 forward under posterior border of mandible, so that the ledge beneath 

 the pit is broadly rounded off. The pit is small and ill-defined, and 

 the region which it occupies is carried so far backward by the very 

 gradually sloping symphysis that its upper margin lies at level of 

 posterior premolar. In the region of temporal muscle-insertion the 

 ridge extending upward toward the coronoid process is usually de- 

 flected forward below the base of the process. The dental foramen 

 and the region behind it are about as in Pongo. The strengthening 

 "ridge of articular process is more evident than in Pongo but less 

 defined than in Pan. 



The lower molars in the living primates represent three main types 

 of structure, peculiar respectively to: (a) the American monkeys, 

 (b) the Hylobatidce, great apes, 1 and Hominidce, and (c) the remain- 

 ing Old World forms. The first type (most clearly shown by 

 Alouatta) is essentially that of the more primitive lemur molars (as 

 in Propithecus) modified by partial or complete suppression of the 

 paraconid and by various degrees of flattening out of the original tri- 

 angles, with no addition of new elements. In the second type the 

 paraconid is absent (sometimes a faint trace in Gorilla) and there is 

 normally a well-developed talonid. The posterior half of the crown 

 is, as in the first type, basin-shaped; and any transverse ridge which 



1 Also in the extinct genera Dryopithecus and Sivapithecus. 



