624 THE EOCENE FAUNA. 



the posterior external cone is elevated. There is a trace of the posterior 

 internal. There is also an anterior ledge. The heel of the third premolar 

 rises to a median blade and posterior cusp. The anterior cusp is elevated 

 and compressed, and supports a small internal lateral cusp. The base of 

 the crown of the third premolar is elongate. All the teeth are rather com- 

 pressed, and there is only a trace of an external cingulum. 



The ramus is compressed and moderately deep. The dental foramen 

 is large, and its superior border is on a level with the posterior base of the 

 crown of the third true molar. Its inferior base is in line with the base of 

 the crown of the second true molar. 



Measurements of mandible, 



M. 



Length of last s-x molars 0o30 



Length of true molars 0310 



( anteroposterior 0065 



Diameters third premolars transverse 0040 



(vertical 0052 



t anteroposterior 009J 



Diameters second true molar < transverse 0000 



( vertical 0062 



T^- _ i 4.1 ■ 1 t 1 S anteroposterior 0120 



Diameters third true luolars ' '■ 



(. transverse OOfaO 



Depth of ramus at Pm. iii 0170 



Depth of ramus at front of M. iii 0220 



The nearest ally of this species outside of the genus Systemodon is 

 probably the Hyracotherium craspedotmn Cope. This species was brought 

 from the Wind River bad lands, and does not occur in the Big-Horn col- 

 lection. It is about the size of the -S'. semihians, but is a true Hyracotherium, 

 with a diastema behind the fii'st premolar. The strong cingulum which 

 characterizes it is not found in the 8. semihians, and the inferior molars are 

 wider and more robust. 



HYRACOTHERIUM Owen. 



Transactions of the Geological Society of London, 1841, pp. 203-208. British Fossil Mammals, pp. 419- 

 423. Gervais Paleontologie Fraufaise, p. — . Cope, Report U. S. Geol. and Geog. Expl. Surv. 

 W. of 100th Mer. Capt. G. M. Wheeler, 1877, iv, pt. ii, p. 2ri8. 



Dental formula, I. -; C. -; Pm. ^; M. 1 Three diastemata in the 



O 1 4: O 



upper jaw, i. e., behind the third incisor, the canine, and the first premolar, 

 respectively; two in the lower jaw, behind the canine and first premolar, 



