102 THE MAMMALIA OF THE DEEP RIVER BEDS. 



decided sigmoid curvature, when viewed from the side, than in the recent type. The 

 intertrochlear ridge is better developed than in the animals from Sansan and the 

 intertrochlear furrow not so deep ; the borders of the anconeal fossa are much more 

 prominent and directed more obliquely towards the inner side and the inner margin 

 is nearly vertical and parallel to the long axis of the shaft, as in the horse. The 

 supinator ridge is less prominent than in the latter. Altogether, the humerus of A. 

 equinum, both in its proportions and in the details of its construction, approximates 

 the modern type much more decidedly than does that of the European species. 



The ulna is quite different from that of the last-named species, in some respects 

 being more equine and in others less so. The shaft is much reduced, but it is not 

 interrupted and at no point is there any coossification between the ulna and radius, 

 though doubtless this feature is subject to variation, as it is in Mesohippus. In order 

 to make clear the differences between the two species of Ancliitherium, with regard to 

 the structure of the ulna, it will be best to give Kowalevsky's description of it in the 

 French species, which is essentially as follows : The olecranon is much compressed 

 and resembles in general that of the Palseotheria in the absence of that curvature 

 towards the inner side which is characteristic of the horses. As in the latter, the 

 sigmoid facet is not continuous, but is interrupted on the external border by a deep 

 sulcus. The proximal radial facet is continuous, not interrupted in the middle. For 

 the distal 40 mm. of its course the ulna is coossified with the radius, but only slightly 

 so, for among thirty specimens which Kowalevsky examined there was but one in 

 which the distal end of the ulna was still attached to the radius. 



In A. equinum the olecranon has the inward curvature which is found in the 

 horses, but not in so marked a degree, and the process rises more steeply and pro- 

 jects less decidedly backward ; the sigmoid facet is not interrupted upon the external 

 margin and especially in the distal portion the humeral surface is very much larger. 

 The radial facets are of very unequal size, the external being much the larger, and 

 the two are nearly but not quite separated by an emargination of the inferior border. 

 The transverse width of the ulna, measured across the radial facets, is very much 

 greater relatively than in the horse. The radio-ulnar arch is as considerable as in A. 

 aurelianense, but distal to this the two bones arc in contact throughout their entire 

 length, as, according to Kowalevsky's figure, they are not in the European species. The 

 distal portion of the bone is also very different from what we find in that species; 

 the lower part of the shaft expands into quite a broad plate, which is received into a 

 deep notch in the radius ; beneath this, the shaft abruptly contracts, expanding again 

 distally to form the carpal surface. The latter is also different from that of the 

 European form, where it is triangular with its greatest diameter directed transversely, 



