THE MAMMALIA OF THE DEEP RIVEll BEDS. 



83 



posteriorly and broadened transversely. The distal end, however, retains its trihedral 

 shape and the carpal facets differ in no important respect from those of the White 

 Eiver form. The distal end of the ulna is coossified with the radius to about the 

 same extent as frequently, though not always, occurs in the latter. 



The carpals exhibit numerous differences of detail from those of Mesohippus, 

 many of which are to be correlated with the increase in size of the median digit. In 

 general, the most noticeable change in the carpus is the increased breadth and 

 decreased proportionate height of its elements. The scaphoid is broader and deeper 

 (antero-posteriorly) relatively to its height, and on the radial surface the anterior 

 ridge is higher and the posterior concavity deeper. The external contour of the rad- 

 ial facet is more deeply notched. The lunar differs in the greater width of the dis- 

 tal as compared with that of the proximal end, and the more nearly square outline of 

 the radial surface, which is less contracted posteriorly. The magnum facet is also 

 carried farther back. In these respects the lunar of the species before us is less 

 equine than that of Mesohippus. On the other hand, it is more modernized in the 

 greater breadth of the surface for the magnum in proportion to that for the unciform 

 and in the open angle at which the two facets meet. The pisiform is more equine in 

 being more expanded vertically and of more uniform height, contracting less towards 

 the proximal end. The cuneiform facet is more oblique, presenting less downward 

 and more forward, but is still only imperfectly divided into two parts. The trape- 

 zoid is very similar in the two genera. The magnum is much broader, especially 

 anteriorly ; this extension is chiefly towards the radial side and consists principally in 

 a broadening of the scaphoid surface. Distally, we find the expansion for the median 

 metacarpal more symmetrically formed on both sides of the posterior prolongation. 

 The head of the magnum remains, as before, very narrow. 



In the metacarpus the only noteworthy change is the expansion of the median 

 and reduction of the lateral digits ; in consequence of this, the magnum surface on 

 mc. iii is decidedly changed in shape. The other carpal facets of the metacarpus 

 remain very much as before. Mc. ii has the same connections with trapezium, trape- 

 zoid and magnum, and mc. iii with magnum and unciform, the latter facet being 

 divided into two parts by a sulcus, which is somewhat better marked than in Meso- 



liippus. 



Measurements, m. 



Breadth of humeral trochlea • • • " ° 35 



Length of radius . ....... • 159 



Breadth of proximal end 031 



Breadth of distal end 038 



Width of head of mc. iii • • • • • - 016 



Depth of head of rnc. iii 014 



