MUSEUM OF COMPARATIVE ZOOLOGY. 89 



the eleven elements which compose the upper Equus molar, we find that six 

 belong to the primitive sextubercular bunodont crown. Two elements of the 

 ectoloph, the anterior pillar and median pillar, rise I'njm the simple primitive 

 basal cingulum of tlie Hijracothcriain molar ; the same mode of devL'lo}imiint, 

 we have just seen, is true of the pjosterior 'pillar. Tlie eleventh el'Uinit, the 

 fold of the postero-external angle of the crown,]), is not prominent until we 

 reach Equus. Tlie term "posterior pillar" is taiven from Lydekker ; the other 

 terms, " median " and " anterior," are applied to parts which have an analogous 

 origin from the basal cingtilum. The remaining coronal cusps are readily iden- 

 tified with their honiologues in the primitive tritubercuhir mular. 



? Anchitherium parvulus, Marsh. 



(Syn. Equus parvulus, Marsh.) 



Among the Loup Fork specimens collected by Clifford are found two lower 

 molars, m-^ and m^, which are almost identical in size with those of Mesohippus 

 Bairdii. The crown of m^ measures: antero-posterior, .011 m. ; transverse, 

 .009 m. Unlike the Mesohippus molars, there is no external cingulum. The 

 " posterior pillar " has the same degree of development as in Ancliilherium. The 

 fangs are separate. There is no trace of cement. iMarsh has described a di- 

 minutive horse (Equus parvulus), estimated at two feet in heiglit, from the 

 same beds, and it is highly probal)le that these teeth belong to this species. The 

 generic reference is of course very tmcertain. The brachydont crowns jjoint 

 either to Menjchippvs or Anchitherium, Ijut the stage of development of the 

 coronal pattern approximates most closely that in the latter genus, being a little 

 more advanced than in Mesohippus. 



RHINOCERID^. 

 ACERATHERIUM, 



The Manus and Pes. 



The characteristics of the pes of Hyracodon from the lower White River beds 

 have been fully enumerated by us.^ They are ])rincipallv as follows: cnboid 

 not supporting astragalus anteriorly ; lateral digits reduced and not spreading ; 

 ectocuneiform not articulating laterally with mts. II. We may subsequently 

 find that the feet of the later species of Hyracodon varied in some of these re- 

 spects, although this is not probable, owing to the fixity of foot-types once 

 established. We have, however, no ])resent means of distinguishing between 

 the Metamynndnn and Accratherium foot-bones. 



On page 1(59 of the first Bulletin a high, rather slender tarsus was descrilied, 



1 See Scott, E M. Museum Bulletin, No. 3, May, 188.3, p. 10. Also, Oshorn, Mam- 

 malia of the Uinta Formation, May, 1889, Part IV. "Evolution of tlie Ungulate 

 Foot," p. 549. 



