[lambe] fossil horses OF THE OLIGOCENE 46 



A right upper molar collected by the writer in 1904 is referred to 

 this species. The upper molar described by Cope is imperfect, the 

 outer slope of the ectoloph is missing and the anterior part of the tooth, 

 including the protoloph, is much damaged. 



The crown of the molar obtained last summer is practically perfect, 

 and has been subjected to little use during the life of the animal; it is 

 regarded as the second molar, and is shown in figures 1, la-c of plate II. 

 This tooth is brachyodont, with well developed low cross crests (proto- 

 loph and metaloph). The crown, seen from below, is suboblong in out- 

 line, transversely broader in front than behind, and relatively narrow 

 in an antero-posterior direction. The outer border (ectoloph) rises 

 higher than the cross crests. The latter are unequal in length, the 

 protoloph being longer and better developed than the metaloph. The 

 intermediate cusps (protoconule and metaconule) are both well defined, 

 although the protoconule is larger than the metaconule, and more dis- 

 tinctly separated from the protocone than is the metaconule from the 

 hypocone. The protocone is slightly larger at its base than the hypo- 

 cone, but both have about the same height. There is no hypostyle. 

 The parastyle is large and adds considerably to the crown's anterior 

 transverse diameter. The mesostyle and metastyle are distinct, and 

 the ribs are distinguishable, the anterior one being the better defined 

 of the two. The cingulum is well developed, and passes from the meta- 

 style entirely round the inner side of the crown to the parastyle without 

 interruption, except for a short distance on the front inner slope of the 

 protocone; it connects in front with the parastyle, with which the outer 

 end of the protoloph shows a marked tendency to unite. Outwardly, 

 the cingulum rises on to the parastyle, but does not cross it. 



il/, westoni, judging of its dental characters principally from the 

 molar obtained last summer, approaches closely to M. iatidens^ Douglass 

 in tooth-structure, but its molars are distinguished principally by the 

 presence of an internal cingulum, by the less pronounced parastyle and 

 a proportionately greater antero-posterior diameter, with the protoloph 

 more nearly equal in length to the metaloph, as well as by other char- 

 acters, il/, celer,^ March and M. montanensis,^ Osborn, are two other 

 nearly related but apparently distinct species from the Lower Oligocène. 

 The presence of the highly developed internal cingulum is one of the 

 most interesting characters in the dentition of il/, ivesioni. This char- 

 acter together with the absence of a hypostyle points to this species 

 being probably the most primitive of the kno^\Ti horses of Oligocène age. 



' Annals of the Carnegie Museum. Vol. II, No. 2, 1903. New Vertebrates 

 from the Montana Territory by Earl Douglass, p. 161, fig. 7. 

 ' American Journal of Science, Vol. VII, 1874, p. 251. 

 * 1904, op. cit. 



