6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



side begins with the molars, while in the great apes it begins with 

 the premolars or canines. A line joining the middle of posterior 

 border of m 2 with the middle of anterior border of m 1 , will, if contin- 

 ued forward in front of incisors, converge rapidly with the sagittal 

 line similarly extended (text fig., 1 and 3, b). In the great apes 

 and in most of the monkeys except certain smaller South American 

 forms a line passing through middle of posterior border of itu and 

 middle of anterior border of m, is essentially parallel to the sagittal 

 line (text fig., 2, b) . In the Hominidce the inward curve of the tooth- 

 row normally begins with the first lower molar. The axis of this 

 tooth prolonged backward (text fig., 1 and 3, c) diverges rapidly 

 from a line parallel to the sagittal plane and crosses the posterior bor- 

 der of m, on outer side of middle ; continued still further it passes 

 through the condyle. That of the second tooth similarly prolonged, 

 while diverging slightly from a line parallel to the sagittal plane, 

 passes considerably to inner side of condyle. In all living genera 

 of great apes and in the fossil Propliopithecus, Dryopithecus, and 

 Sivapithecus the axes of the two teeth (text fig., 2, b) lie in one 

 line essentially parallel to the sagittal line and passing further to inner 

 • side of condyle than is the case with the axis of m 2 in the Hominidce. 

 The symphysial region in the Hominidce seldom extends conspicu- 

 ously behind the level of the incisors, and never bears a marked con- 

 cavity on its posterior border for insertion of the lingual muscles; 

 in other primates it always extends conspicuously behind level of in- 

 cisors and it usually bears a marked concavity on its posterior border. 

 The mylohyal ridge is well developed in the Hominidce, but is barely 

 indicated in monkeys and apes. 



While sharing those general peculiarities which distinguish other 

 primates from the Hominidce, the three 1 genera of living great apes 

 are readily separable from each other by the details of their mandibu- 

 lar structure. In Pan and Pongo the digastric muscle is inserted 

 along the lower border of the mandible, rarely extending forward 



1 In the most recent complete work on the order, Elliot's " Review of the 

 Primates," New York (1912), June, 1913, four genera are recognized: Pongo 

 Lacepede for the orangs, Gorilla I. Geoffroy for the gorillas, Pseudogorilla 

 Elliot (1. c. vol. 3, p. 224) for an animal supposed to be the Gorilla mayema 

 of Alix and Bouvier, and Pan Oken for the chimpanzees. The genus 

 "Pseudogorilla" was based on two specimens of true Gorilla, an immature 

 male with all the teeth in place but with the basal suture open and the tem- 

 poral ridges separate (1. c. pi. 32), and a mature female with the basal 

 suture closed and the temporal ridges joined (1. c. pi. 33). Three valid 

 genera are thus left in the group. 



