282 Thorpe — New Genus of Oligocene Hycenodontidce. 



the three acting in unison were powerful. They are all 

 situate near the glenoid, and afford a great arc with com- 

 paratively small muscular contraction and expansion, 

 except in the case of the temporalis. 



The paroccipital process lies considerably posterior to 

 the glenoid, and furnishes the digastric muscle with an 

 origin well adapted for wide jaw gape. The mastoid in 

 Neohycenodon is very small and located on a line with the 

 anterior part of the occipital condyles. In contrast to 

 Smilodon, its small size must indicate a rather weak 

 development of the cleido-mastoid and sterno-mastoid 

 muscles, the chief function of which is to pull the head 

 downward upon the neck. 



Comparison with Dinoceras. — Although Neohycenodon 

 and Smilodon are of widely separated geologic horizons 

 and of different orders, yet they were both carnivorous in 

 habit and had developed skull characters of great sim- 

 ilarity. Now let us examine a representative of another 

 group, totally distinct in genetic relationships and 

 habits from either of the above, namely, Dinoceras, sev- 

 eral skulls of which in the Marsh Collection have served 

 for comparison. This genus possessed long superior 

 canines and was therefore compelled to have a wide jaw 

 gape. In this form the peculiar glenoid articulation is 

 shown, although it has not descended below the basicran- 

 ial plane. The temporalis muscle was long and of small 

 leverage, while the zygomatic arch is apparently rela- 

 tively smaller than normal in herbivorous animals. The 

 condyle of the ramus* shows a peculiar modification, as 

 well as the outward curvature of the whole ramus just in 

 advance of the condyle, to permit of a wide gape. An 

 interesting development of the ramus is the hoplopho- 

 neoid character shown in the long decurved processes for 

 the protection of the superior canines when the mouth is 

 closed. Protoceras is analogous to Dinoceras in these 

 jaw mechanics. 



The primal cause for the necessity of a wide jaw gape 

 is great canine length. Smilodon and Dinoceras pos- 

 sessed very short inferior canines relative to the great 

 length of the superior, whereas the upper and lower 

 canines of Neohycenodon were of more nearly equal 

 length. In Dinoceras mirabile the combined length of the 

 upper and lower canines is 195.5 mm., the superior being 



