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 from the simple primitive 

 basal cingulam of the Hyracotherium molar ; the same mode of development, 

 we have just seen, is true of the posterior pillar. The eleventh element, the 

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

 reach Equus. The term " posterior pillar " is taken from Lydekker ; the other 

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

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

 tified with their homologues in the primitive tritubercular molar. 



? Anchitherium parvulus, Marsh. 



(Syn. Equus parvulus, Marsh.) 



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

 molars, m x and ra 3 , which are almost identical in size with those of Mesohippus 

 Bairdii. The crown of m 1 measures: antero-posterior, .011m.; transverse, 

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

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

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

 minutive horse {Equus parvulus), estimated at two feet in height, from the 

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

 generic reference is of course very uncertain. The brachydont crowns point 

 either to Merychippus or Anchitherium, but 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^L 

 ACERATHERIUM. 



The Manus and Pes. 



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

 have been fully enumerated by us. 1 They are principally as follows : cuboid 

 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 present means of distinguishing between 

 the Metamynodon and Aceratherium foot-bones. 



On page 169 of the first Bulletin a high, rather slender tarsus was described, 



1 See Scott, E M. Museum Bulletin, No. 3, May, 1883, p. 19. Also, Osborn, Mam- 

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

 Foot," p. 549. 



