232 EOCENE FAUN^. 



posterior basin-like heel is wide, and is bounded on the external side by an 

 angular crest. A cusp rises from the posterior inner angle, which is sepa- 

 rated from the anterior cusp by a deep notch. There is a small posterior 

 marginal tubercle at its external base. The heel of the fourth premolar is 

 wide, and has a low cusp at each posterior angle. There are but two ante- 

 rior cusps, the external of which is the larger, and is situated anteriorly to 

 the well develo^ied inner one. The last inferior molar is lost from this spec- 

 imen. The mandibular ramus is compressed and rather deep, and becomes 

 deeper anteriorly. The symjihysis begins below the fourth premolar and 

 was not coossified, although the animal is clearly adult. 



Measurements of mandible. 



If. 



Length of posterior four molars 0160 



Length of true molars 0120 



Length of base of last molar 0046 



Diameter, m. ii ^ ''"*"''P°«'«"°'' !^^^ 



t transverse 0030 



Diameters Pm. iv J ''""^■•°P''«'«"°'^ f^l 



(. transverse 0028 



Depth ramus at Pm. iv 0078 



Depth ramus at M. iii 0086 



Associated with some of the mandibular fragments, but not known to 

 belong with any in the same skeleton, is the fragment of upper jaw above 

 mentioned. It supports the first and second true molars, in perfect preser- 

 vation. They dififer from those of Hyopsodus vicarius (Plate — , fig. — ), 

 which are of about the same size, in their rather more triangular form. This 

 is due to the reduction of the posterior inner tubercle to a mei'e ledge sup- 

 porting a small cusp. The anterior cusp is prominent, and is connected by 

 ridges with two small angular tubercles on the anterior and posterior mar- 

 gins of the crown. The external lobes stand on the external border of the 

 base, are distinct, acute, angular, and with lenticular section. Their exter- 

 .nal base is bounded by a cingulum which rises into a low cusp opposite the 

 interspace between them. Slight anterior and posterior cingula. 



Measurements. 



Length of base of M. i ami M. iii 0070 



I anteroposterior 0040 



Diameters M. ni, transverse 0045 



( vertical 0020 



