118 THE MAMMALIA OF THE DEEP RIVER BEDS. 



A. equinum has lost the structure in question, as is the case with certain horses from 

 the Pliocene of Florida, to which Cope has called attention, or, as is more probably 

 true, the American species never acquired the character. Prof. Cope very kindly 

 allowed me to examine his beautiful series of specimens of Mioliippus from the John 

 Day beds of Oregon, and in all of them I found that, while the invagination was 

 fairly well marked in the upper incisors, it was not indicated at all in the lower. 

 (2) The sigmoid facet of the ulna is not continuous, but interrupted on the external 

 side by a deep sulcus. (3) The facets on the pisiform for the cuneiform and ulna, 

 respectively, are widely separated. (4) In the specimens from Sansan (but not those 

 from Steinheim, which perhaps should be referred to a different species) the median 

 ungual phalanges are less depressed and flattened and those of the lateral digits are 

 decidedly more reduced in size than those of the American species. (5) The keel 

 on the distal trochlea of the median metacarpal, and the corresponding groove on 

 the proximal phalanx, are more extended anteriorly. This appears not to be true of 

 the specimens from Steinheim, and doubtless, as Kowalevsky has suggested in the 

 case of ffipjxxrion, the shortening of the lateral digits is causally connected with the 

 increased size and importance of the metapodial keels. 



The evidence here brought forward seems to lead us to the following conclusions. 

 The genus Anchitlierium, in the restricted sense of the term, is of American origin 

 and reached Europe by migration. It cannot be regarded as a member of the direct 

 ancestry of the modern JEquidce, but as a side branch of that stem, which was prob- 

 ably derived from some of the John Day forms not as yet identified. Though 

 appearing later in time than these forms, it nevertheless is in some respects more 

 widely removed from the recent horses than they. This is notably the case in the 

 dentition, where the "pillars," and especially the anterior ones, of the lower molars 

 and premolars, and the posterior conule of the upper, appear to be undergoing a 

 retrogressive metamorphosis. Further, in no species of Mioliippus, or even of Meso- 

 Jiipjms, are the lateral metapodials so large as in the Steinheim form of A. aurelian- 

 ense. Some of the John Day species have a distinctly more modernized type of 

 skull than any species of Anchitlierium. In M. protistans, for example, the orbit is 

 very far back, shifted almost entirely behind the line of the molars. "While the skull 

 of A. equinum has great vertical depth in the orbital region, the orbit remains very 

 low in the face and the zygomatic arch descends very abruptly in its passage for- 

 ward. Though we have still, it is obvious, much to learn as to the exact relationship 

 between Ancliitherium and Mioliippus, the general position of the former with refer- 

 ence to the main equine stem is now reasonably clear. 



Taking now a broader view of the series of equine genera which have been 



