F. B. Loomis — Hyopsodldce. 417 



Two genera are distinguished by Cope and Osborn on the 

 condition of the metaconid : when simple, Hyopsodus : if bifid, 

 Sarcolem/w*. The two genera are certainly distinct but farther 

 characters are required to separate them ; for several jaws, 

 which, in all other features, are most closely related to Ilyop- 

 sodus, still have the metaconid bifid. This character has also 

 been noticed as occurring in the genus Eoliippus, Plienacodas, 

 and Systemodon, and is not, therefore, considered alone enough 

 to separate the two genera. The bifid metaconid is used below 

 to distinguish some of the species of Hyopsodus. 



Hyopsodus is characterized by lower wider teeth, the cusps 

 being blunter ; the fourth premolar is wide and not compressed, 

 having fewer and less developed cusps. It occurs in the 

 Wasatch, AYind River and Bridger horizons. In several species 

 there is a tendency to have the metaconid bifid, but it is never 

 as marked as is characteristic of Sarcolemitr. 



Sarcolemur is characterized by narrow trenchant teeth, with 

 pointed cusps ; the fourth premolar is much compressed and 

 has the anterior cusps well developed. It is as yet known 

 only from the Bridger, and of the promolars only the fourth 

 is known. 



Hyopsodus Leidy 

 Lemuravus Marsh, Stenacodon Marsh, JMicrosus Leidy, Diaco- 



dexis, Cope. 



The genus was founded by Leidy* on an imperfect lower 

 jaw of H.pcmlus from the neighborhood of Ft. Bridger. The 

 name Microsusf was given the same year to a second species 

 differing only in the proportionate depth of the mandibular 

 ramus. Stenacodon^: was founded by Marsh on what proves 

 to be a last lower molar, the genus being separated on the 

 ground that there was no cingulum. Lemuravus§ was proposed 

 for a species on the basis of the incisor formula being 3/3 

 which, later, proved to be true for Hyopsodus. Diacoclexis\ 

 was proposed by Cope for a specimen, including premolars of 

 EoMppus. As yet the only members of the family found in 

 the AYasatch and Wind River horizons belong to the genus 

 Hyopsodus, the skeletal features of which are as follows : 



The skull is elongate with a level top terminating abruptly 

 behind, the rear of the cranium overhanging the occipital con- 

 dyles, somewhat as in Rodent skulls. The zygomatic arches 

 are slender but widely arched. On the deep lower jaw, the 

 anterior border of the surface for attachment of the masseter 



* Proc. Acad. Nat. Sci., Phila., 110, 1870. 



f Same cit., p. 113. 



% This Journal, ii, 210, 1892. 



§ Same cit., vol. iv, p. 239, 1875. 



I Proc. Amer. Phil. Soc, vol. xxi, 181, 1883. 



Am. Jour. Sci. — Fourth Series, Vol. XIX, No. 114.— June. 1905. 

 29 



