HYEACOTHERIUM. 647 



of Wyoming. A considerable jjart of the skeleton was extracted from the 

 matrix by Mr. Wortman, and other parts were disengaged after their arrival 

 in Philadelphia. The mandibular ramus of the second individual agrees in 

 characters with those of the type specimen. 



Hykacotherium osbornianum Cope. 



Orotherimn sylvatieum Cope. Annual Report U. S. Geol. Survey Terrs., 1872 (1673), p. 607. Hyracotlus- 

 riiim siihaticiim Cope. U. S. Expl. Surv. W. of 100th Meridian. G. M. Wheeler, IV, II, p. 262; 

 not Lopiotherium sylratictim Leidy. 



Plate XXIV; fig. 23. 



This species is represented by a left mandibular ramus, which supports 

 more or less of five molar teeth. The proportions of the jaw are about 

 those of H. anyustidens, but are rather more slender than the smallest of that 

 species that are known. On the other hand, the molar teeth are of larger 

 size than in any of the varieties of that species. This combination of char- 

 acters indicates a species distinct from the H. angustidens, a fact consistent 

 with the diversity of the horizons in which the two species are found. 



The sides of the ramus are very flat, the outer side even a little con- 

 cave along its middle, and neither side has any prominent convexity along 

 the alveolar borders. The massetei'ic fossa is scarcely concave, and is only 

 indicated by a little swelling of the external border of the ascending ramus. 

 There is a mental foramen beneath the posterior root of the third premolar. 

 The first true molar is the only one in which the grinding surface is well 

 preserved. The external Vs are well defined, and their angles are directly 

 opposite the internal cones. The latter are both simple, and have a round 

 section. There is a strong anterior basal cingulum, which is joined at the 

 middle by the anterior horn of the anterior V. There is an uninterrupted 

 external basal cingulum, and also a posterior one which is joined by the 

 posterior branch of the posterior V, close to the posterior inner cone. 



Enamel smooth. 



Measurements. 



M. 



Length of posterior four molars 034 



Length of fourth premolar 007 



Length of third true molar 010 



Width of third true molar anteriorly 004 



Depth of ramus at front of last true molar 014 



