Cope.] 186 [Dec. 16, 



The nearest ally of this species outside of the genus SysUmodon is prob- 

 ably the Uyracotherium craspedotum 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 Hyracotheri- 

 v,m, with a diastema behind th(5 first premolar. The strong cingulum 

 which characterizes it is not found in the S. semihians, and the inferior 

 molars are wider and more robust. 



Hyracotherium craspedotum: Cope. 



Bulletin U. S. Geol. Survey, Terrs., 1881, p. 199. American Natural- 

 ist, 1880, 747. 



The dentition of this species is in its dimensions and proportions inter- 

 mediate between the two species of Systemodon, Its thi'ee premolars 

 equal four of those of the S. semihians, while the molars of the two 

 species are about equal. 



A specimen having the proportions of the H. crnspedotum was found by 

 Mr. Wortman on the Big-Hoi'n, but unfortunately it does not exhibit the 

 characteristic cingula of the two dental series. The second superior pre- 

 molar, like that of Systemodon semihians has no internal tubercle. It is 

 not certain whether there is any diastema posterior to the first superior pre- 

 molar. I therefore cannot yet ascertain whether this specimen represents 

 an undescribed species of Systemodon or Hyracotherium, or a strong variety 

 of the H. craspedotum. The accompanying inferior true molai-s are inter- 

 mediate in size between those of the latter species and the H. vasacdense. 



Hyracotherium vasacciense Cope. 



This species differs from the H. mnUcolum'm its deep mandibular ramus. 

 A single specimen from the Big-Horn presents the same proportions. The 

 posterior inferior molar is rather short. 



Hyracotherium venticolum Cope. 



Bulletin U. S. Geol. Survey, Terrs., 1881, 198. 



Fifteen individuals of this species are included in the collections. 



Hyracotherium angustidens Cope. 



This was a very abundant species. Mr. Wortman's collection contains 

 jaws and teeth of twenty individuals sufficiently well preserved for 

 identification, and a large number of other pieces of jaws, etc., which may 

 be reasonably inferred to belong here. 



In my report on the Wind River collection* I noticed three varieties of 

 this species, which differ in the depths of the ramus at the line of junction 

 of the fourth and fifth molars. The numbers are 12, 14, and 15.5 mm. 

 respectively. The lengths of the first true molar also vary from 7 to 6.5 

 and 7.5 mm. respectively. The last true molar measures in all 10.0 mm. 

 The majority of the Big-Horn specimens agree with the second variety, 

 but two others occur, one a little smaller, and the oilier a little larger 

 than the average. The former measures ; length of last molar .0090 ; of 



* Bulletin U. S. Geol. Survey Tens, vi, 1881, p. 108. 



