Thorpe — New Genus of Oligocene Hycenodontidce. 283 



176 mm. long; in Smilodon calif ornicus the total length is 

 174 mm., with the superior canine 154 mm., while the total 

 length in Neohycenodon horridus is 100 mm., about equally- 

 divided between superior and inferior canine. Another 

 point of comparison is in the mesaticephaly of Smilodon 

 as compared with the dolichocephaly of Dinoceras and 

 Neohycenodon. Still another point to consider is that the 

 condyle of the ramus is situated a little below the superior 

 edge of the alveolar parapet in Smilodon and in Neohy- 

 cenodon, but on a line with the tooth-row in Dinoceras. 



In short, we see in these genera how a certain specific 

 end-result — the accomplishment of great jaw gape — was 

 attained by three different modes of skull and jaw 

 mechanical modifications. In the three genera there are 

 similar modifications, but the divergences are equally well 

 marked. In Neohycenodon the basicranial area is fore- 

 shortened, the glenoids are lowered and the mandibular 

 condyle is but slightly above the angle. In Smilodon the 

 basicranial region is not foreshortened, but the cranium 

 elevated, the glenoids lowered, relative to the basicranial 

 plane but not to the palatal, while the mandibular condyle 

 is likewise low. Smilodon evidently was the only one of 

 these genera under consideration that struck downward, 

 hence the form of the skull. In Dinoceras the basicranial 

 region is somewhat foreshortened, to a lesser extent than 

 in Neohycenodon, while the glenoid has not lowered, but 

 the mandibular condyle has become elevated with respect 

 to the tooth-row. 



Hycenodon Laizer and Parieu 1839. 



Hycenodon cruentus Leidy 1853. 



A skull and anterior part of both rami (Cat. No. 12762, 

 Y. P. M.) of this species were collected near Red Cloud, 

 Nebraska. It is a young individual, with the permanent 

 canines just beginning to appear. The lower incisors 

 show that the first was not more than half the size of the 

 second, which was nearly as large as the third. The two 

 second incisors have their usual position posterior to the 

 line of the others and are separated from each other by a 

 diastema of only 1.5 mm. 



Another specimen, Cat. No. 12764, Y. P. M-, from 

 Chadron, Nebraska, has. but slightly worn dentition and 



