THE MAMMALIA OF THE DEEP RIVEK BEDS. 



103 



while in the American species, as in the horse, it is much deeper antero-posteriorlj 

 than broad transversely and projects behind the lunar facet of the radius. The 

 external side displays no such tendinal sulcus as in the horse; the pisiform facet is 

 narrower than in that animal but relatively higher. 



The radius is, in many respects, more like that of the modern type than is the 

 same bone in A. aurelianense, but still retains a number of primitive features. The 

 proximal end is expanded transversely, though somewhat less so than in the horse, 

 a difference which is partly due to the much smaller size of the tuberosity for the 

 attachment of the external lateral humero-radial ligament. The bicipital tuberosity 

 occupies very much the same position as in Equus, but is reflected somewhat more 

 upon the internal face and the internal ligamentous process is more prominent. The 

 humeral facets are very similar in the two genera, but the intertrochlear ridge is nar- 

 rower in the extinct animal, and the deep sulcus which in the horse invades this ridge 

 is, in A. equinum, represented by a small raised surface with roughened borders. The 

 intertrochlear furrow is well marked, though less so than in Uquus, and produces a 

 shallower emargination of the anterior rim. The facet external to this groove is 

 relatively broader than in the modern genus. The shaft is of very uniform dimen- 

 sions throughout; it is slightly arched forward, broad, and antero-posteriorly com- 

 pressed, and in general very similar to that of Uquus, but is more slender and 

 rounded, less expanded and more trihedral distally, where the inner face forms an 

 angle with the anterior, instead of curving gently into it. This trihedral shape is 

 found in all of the primitive equines and even persists in Desmatippus. The sulci 

 for the extensor tendons are narrower and have less rugose and elevated margins 

 than in Equus. The postero-external angle of the shaft forms, for most of its length, 

 a roughened ridge, to which the ulna is closely applied, and just above the distal end 

 is deeply notched in order to receive the expansion of the ulnar shaft already referred 

 to. Beneath this notch the radius expands to its maximum distal breadth and then 

 narrows again to the carpal surface. The latter is almost exactly as in M caballus, 

 the following being the only differences which can be observed : (1) The scaphoid 

 facet is more concave in front and descends more abruptly behind ; (2) the same 

 facet narrows at the posterior projection behind the plane of the lunar surface, instead 

 of being of nearly uniform width ; (3) there is no facet for the lunar upon the ulnar 

 side of this scaphoid projection ; (4) the lunar facet is not reflected so far upon the 

 posterior side of the bone. 



From this description it will at once be evident that the radius of J., equinum 

 approximates that of the modern forms very closely, and thus departs considerably 

 from A. aurelianense, which displays this modernization in a less degree, as may be 

 a. p. s. — VOL. XVIII. sr. 



