200 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.yi. 



These indicate an animal of much smaller size than the P. primcevus, 

 but with a similar constitution of the molar teeth. The teeth support 

 four conic cusps, which do not inchne to fuse transversely, as is gener- 

 ally the case in Syracotherium. Those of the posterior pair are sepa- 

 rated by a tubercle, and a rudimental tubercle stands behind the notch 

 between those of the anterior pair. The posterior median tubercle 

 is developed into a heel on the last molar. On prolonged wear, the re- 

 sulting pattern represents two Vs, the posterior limb of the posterior 

 being regularly convex backwards. There are no cingula on any of the 

 molars. The ramus is rather robust and is not deep. 



3feasurements. 



No. 1. 



M. 



Length, of bases of posterior live molars ■- 0440 



Length, of bases of true molars 0250 



Depth of ramus at Pm. Ill 0160 



Depth of ramus at M. Ill in front 0190 



No. 2. 



Diameters of M. II 5 anteroposterior 0080 



( transverse 0070 



Diameters of M. Ill 5 anteroposterior 0080 



(transverse -. 0055 



The characters of the typical specimen are as follows : The jaw frag- 

 ment indicates an animal of about the size of the Hyracotherium eraspe- 

 dotum, but with the opposite cones of the inferior molars not united by 

 cross-crests. There is a tubercle between the posterior pair of the first in- 

 ferior true molar. The anterior tubercles of the fourth premolar are close 

 together, and there is a strong cusp anterior to these. No basal cingulum 

 on this tooth. Length of molars 3 -f 4 -f 5, .025 ; depth of ramus at 

 Pm. IV, .018. 



44. Phenacodus purM^vus Cope. 



A fragment of a lower jaw, with teeth. 



45. Phenacodus trilobatus Cope sp. nov. 



A lower-jaw fragment, supporting the three true molars, of one indi- 

 vidual, represents this species. It is of the dimensions of the P.prim(s- 

 vus, and displays the same general constitution of the teeth. The only 

 difference noticeable is an important one. The anterior internal tuber- 

 cle is accompanied by two others of less elevation — the one immediately 

 anterior, the other immediately posterior to and not deeply separated 

 from it. The internal face of these tubercles slopes obliquely outwards 

 on the second and third true molars. The external tubercles have their 

 external faces sloping inwards on all the true molars. There are no 

 cingula. 



