THE MAMMALIA OF THE DEEP RIVER BEDS. 117 



both, this would not diminish the value of either for understanding the systematic 

 position of the other, but would rather enhance it, for this would render more intelli- 

 gible the fact that, on comparing the two species, we find now one and now the other 

 more closely approximating the modern standard, and again, both agreeing in some 

 advance beyond Miohippus, either towards or away from the modern horses. 



As examples of these parallelisms, the following may be selected. (1) In the 

 upper cheek-teeth of Miohippus the posterior transverse crests are separated from the 

 external wall, while in Ancliitlierium they have become coalesced with it. This also 

 happens in the direct line, beginning with Desmatippm. (2) In the upper incisors 

 the pit or "mark" is much better developed than in the John Day form. (3) The 

 orbit has been shifted farther back than in most of the species of Miohippus, though 

 not so far as in M. ])rce.itans. (4) The size of the animal has greatly increased, and 

 the changes which accompany augmented size and weight, such as heavier limb 

 bones with better developed processes for muscular and ligamentous attachments, are 

 well shown. (5) In Miohippus the spout-like shape of the odontoid process of the 

 axis is but barely indicated by a slight elevation of its lateral margins, while in An- 

 chiiherium this character is much more pronounced and is even carried somewhat 

 farther than in the horse. (6) The median digit in both manus and pes is enlarged, 

 and in consequence the magnum in the carpus and the ectocuneiform in the tarsus 

 have become relatively broader and lower. In these respects the two species agree, 

 in others they differ as to the degree of approximation to the modern standard. 



In the following respects A. equinum is more modernized than A. aurelianense. 

 (1) The base of the cranium is more elevated above the plane of the molar teeth, and 

 this brings about an increase in height of the ascending ramus of the mandible and 

 the mandibular condyle is raised higher. (2) The humerus is much more equine in 

 structure, the external tuberosity being reduced in size, the bicipital tubercle much 

 enlarged, median in position and dividing the bicipital groove into two parts; the 

 distal end also is almost completely like that of the horse in construction. (3) The 

 radius is more equine in the expansion of the extremities, widening and flattening of 

 the shaft, the shape of the carpal facets, and in the position of the tubercle for the 

 insertion of the biceps muscle. (4) The olecranon of the ulna has the inward curva- 

 ture characteristic of the recent horses; the distal facet for. the cuneiform is also 

 more as in HJquus. 



On the other hand, the European species approaches the recent type in several 

 points of structure more nearly than does the American. (1) The lower incisors 

 have the pit or enamel invagination as well marked as the upper. The exact signifi- 

 cance of this difference between the two sjiecies is not quite clear; it may imply that 



