42 







Measurements. 













H. 



priscidens. 



MAP 



H. planiccps** 



MM. 



H. nebrascensis 



MM. 



Upper molar series, lei: 



igth., 





62 



103 



70 



Upper premolar series, 

 Mi width, (tr.) 



leng 



th 



62 

 23-2 



35 



26 



M 2 " 







24-5 



24 



22 



23 



J 9-5 



135 



36 

 37 



3') 



40 



28 



M 3 " 







26 



M 1 length (a p.) 



M 2 " 







27 



M 3 " 









Pi width (tr) , 









po a 







20 

 21-7 

 23 

 12-3 







P3 « 









p4 " 









P 1 length (a.p.) 









pi " 







16 



16-8 



17-3 







ps " 









pi " 









The anterior end of the mandible of a Hyracodon, obtained in 1904 from the Bone Coulee 

 beds, is of special interest ; it is provisionally assigned to this species. 



The specimen (plate IV, figures 3 and 4) consists of nearly half the left ramus with that 

 part of the right ramus anterior to the second premolar. Premolars 2, 3 and 4 are preserved, 

 as well as the roots of the canines, and of the three pairs of incisors. The alveolus ot the 

 second premolar is partly seen on the right side. 



A striking feature is the long diastema, between the canine and the second premolar, 

 nearly erpial to that of Hyraehyus (H. ayrarius) iu proportionate as well as actual length, and 

 about twice as long as that of Hyracodon nebrascensis,J^e\dy. A basal cingulum is continuous 

 round the entire crown of the premolars, except for a short distance internally, in the second 

 and fourth teeth, posterior to their mid-length. 



Measurements. 



MM. 



Length of premolar series , 44 



Premolar 2: — 



Antero-posterior diameter 13 



Transverse diameter 10 



Premolar 3: — 



Antero-posterior diameter 15 



Transverse diameter . . 12 



Premolar 4: — 



Antero-posterior diameter 165 



Transverse diameter 135 



Depth of ramus in front of premolar 3 28 ■ 5 



Maximum thickness of ramus beneath same tooth 15 



Length of diastema behind canine 20 



Breadth of mandible at mid-length of diastema 21 



Length of symphysis 52 



Thickness of mandible near posterior end of symphysis 14 



* The measurements of the teeth of S. planieept and H. nebrascensis are taken from the "Preliminary account of the 

 Mammals from the White Rivt-r formation, contained in the Museum of Comparative Zoologr" by W. B., Scott and 

 Henry F. Osborn. 1887, p. 171, Bull. Mus. Comp. Zool., Harvard College. 



