162 



Annals of the Carnegie Museum. 



flaring to accommodate the large Py as mentioned above. Anteriorly 

 the two rami form a rather broad square chin which terminates in- 

 feriorly in a heavy rounded swelling. The external surface of the 

 horizontal ramus is quite plane between P2- and the vertical rugosity 

 for the posterior margin of the alveolo- labialis muscles, while internally 

 the surface is convexo-concave supero-inferiorly opposite the molar 

 region. The under border of the horizontal ramus forms a long antero- 

 posterior concavity due to the heavily rounded chin, and to the de- 

 scending angle at the back of the jaw. The mental foramina are 

 located well down on the jaw; one opposite the anterior part of Pg- 

 and the other opposite Py. 



Fig. 



Side view of left lower jaw of Promerycochcerus carrikeri No. 109; 

 5 natural size. 



The depressed appearance of the vertical ramus is due entirely to 

 the unusually small elevation of the ramus above the horizontal line of 

 the teeth. Thus the coronoid process is a mere blunt and short peg, 

 very little higher than the articular condyle, and strongly directed 

 outward. At the base of the antero-internal angle of the coronoid 

 process there is a broad rugose ai'ea for muscular attachments, which 

 is succeeded by an equally broad though smoother area on the postero- 

 internal angle of the same process, which terminates at the anterior 

 border of the articular condyle. The latter is quite broad transversely, 

 but has a small antero-posterior diameter, while internally it greatly 

 overhangs the pterygoid fossa. The latter fossa is extremely large, 

 quite concave, and has many rugose ridges across it for the attach- 

 ment of the internal pterygoid muscle. There is also a prominent 

 ridge and a rugose area for the attachment of the muscles (external 

 pterygoid) on the neck of the articulating condyle. The dental fora- 

 men is of medium size and is placed high up, on a horizontal line with 



