HYRACHYUS. 657 



best developed on the last true molar. The heel of the latter is a low 

 tubercle. The only lateral cingula are situated each at the base of the 

 anterior lobe of the crown. In the third premolar the anterior cross-crest 

 exists but is twisted backwards at the inner extremity. The heel is like 

 that of the fourth premolar, and the anterior cingulum is better developed 

 than in any other molar. 



The ramus is compressed and rather deep for the size of the molars. 

 The anterior border of the masseteric fossa is not well defined. The 

 symphysis is coossified and is contracted at the diastemata. The men- 

 tal foramen is below the anterior root of the first (second) premolar. 



Measurements. 



M. 



Length of inferior moiar series 053 



Length of true molar series 033 



Length of last true molar 014 



Width of last true molar anteriorly 008 



Diameters of fourth premolar! ^"t^^oPo^t^""' 008 



( transverse 006 



Depth of ramus at diastema 017 



Depth of ramus at penultimate molar 025 



Width of symphysis at diastema of No. 2 0115 



The Wind River Eocene of Central Wyoming. J. L. Wortman. 



HYRACHYUS Leidy. 



Hayden's Beport, Geological Survey of Wyoming, 1871, 357. Proceed. Acad. Nat. Sci., Philada., 1871, 

 p. 229. Cope, Proceed. Amer. Philos. Soc, 1873, p. 212. Annual Report U. S. Geol. Survey, 

 1872 (1873), p. 594. Leidy, Report U. S. Geol. Survey Terrs., I, 1873, p. 60. Scott, Spier, and 

 Osborne, Pal. Report Princeton Scientific Expedn., 1878, p. 49. 



Dental formula: I. -; C. -; Pm. ^; M. -. Transverse crests of molars 

 o 1 4 o 



and premolars continuous. Third and fourth j^remolars with two transverse 



crests; first and second with one or none. Inferior true molars with two 



subtransverse crests; premolars without cross-crests, excepting the fourth, 



which has one corresponding with the anterior crest of the true molars. 



Last true molar without heel. Inferior canines forming a continuous series 



with the incisors, but separated from the premolars by a diastema. 



Nasal bones produced posteriorly and anteriorly, contracting the 



anterior nares. Meatus auditorius externus not closed below. Posttympanic 



and paroccipital processes united for part of their length. Mandibular 



42 O 



