Thorpe — Neiv Genus of Oligocene Hycenodontidce. 281 



In other words, the basicranial area is foreshortened and 

 the glenoid surfaces are far below the basioccipital. In 

 Hycenodon, these articular surfaces are practically in a 

 plane with the basisphenoid, that is, they are much higher 

 and more anterior, and the relations are much more typ- 

 ically mammalian and carnivoroid than in Neohycenodon. 



Comparison ivith Smilodon. — This new genus exhibits 

 many details of structure like those in Smilodon, but at 

 present we shall consider only similarities and differences 

 which are due to the mechanics of the jaw movements. 

 For comparison I have used a fully adult skull, Cat. No. 

 10204, Y. P. M., of Smilodon calif ornicus Bovard, from 

 the Eancho La Brea, California. The glenoid articula- 

 tions lie much below the level of the basicranial plane, and 

 the zygomatic arches are short and relatively weak in both 

 Neohycenodon and Smilodon. The lowering of the 

 glenoid permits a wide gape of the jaws before the pro- 

 jection of the angle impinges upon the posterior surface of 

 the postglenoid tubercle. This tubercle in both genera 

 has the same form below the glenoid surface, i. e., it is 

 moderately long internally, and its inferior margin slopes 

 upward externally to the level of the articular surface. 

 Moreover, there is no inflection to the mandibular angle in 

 Neohycenodon, and it does not turn outward to the same 

 extent as in Smilodon, but the former genus did not need 

 to open the jaws so widely as did the latter. 



The coronoid process is rather small and not high, 

 a feature which is correlated with the other changes tend- 

 ing to allow the required great gape. The temporalis 

 muscle, attached to the coronoid, and originating on the 

 occipital and sagittal crests and on the parietal and 

 squamosal bones, was of great length and therefore prob- 

 ably of small leverage, but compensation for this is in part 

 accomplished by the posterior position of the cutting 

 teeth. The masseteric fossa is moderately large and 

 rather shallow, while the zygomatic arch is short and 

 weak, from which it would seem that the masseter muscle 

 had similar power and functions both in the creodonts and 

 in the true Carnivora. The pterygoid muscles, acting 

 mainly with the masseter, originated slightly more ante- 

 riorly than in Smilodon. Of the three sets of muscles, 

 temporalis, masseter, and internal pterygoid, each alone 

 was relatively weak in the power of closing the jaw, but 



