SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 9I 



anconeal fossa is deeper, while the supinator ridge is almost entirely 

 obsolete. 



The fore-arm bones (PI. XII, fig. ii) are separate throughout. The 

 head of the radius is more compressed antero-posteriorly than in Protero- 

 therium and has better distinguished facets for the various parts of the 

 humeral trochlea, while the bicipital tubercle is more prominent; the 

 proximal part of the shaft is likewise more compressed antero-posteriorly, 

 but, for most of its length, the shaft is stouter than in the genus last 

 mentioned ; the distal end, however, is not so thick and the groove on its 

 anterior face for the extensor tendons is much shallower and less clearly 

 marked : the facets for the scaphoid and lunar are more deeply impressed, 

 more distinctly separated, and have a greater dorso-palmar diameter in 

 proportion to their width. 



Of the ulna, we have only the proximal and distal ends ; the olecranon 

 is large, laterally compressed, thickened and slightly incurved at the 

 proximal end ; the proximal part of the shaft is stout and trihedral, but 

 yery probably it was greatly reduced for most of its length, as the distal end 

 is relatively smaller than in Proterotherimn, though it still retains a lateral 

 articulation with the lunar. 



The carpus (PI. XI, fig. 12) is essentially the same as in the tridactyl 

 genera of the family and still remains semi-taxeopod, the scaphoid articu- 

 lating with the magnum, but the lunar excluded from the unciform. The 

 modifications of the carpus, as compared with that of Proterotherium, are 

 less than would be expected from the strictly monodactyl character of the 

 manus, the differences being almost entirely in matters of minute detail. 

 Indeed, there is even less difference in the shape and mutual connections 

 of the carpal elements between Thoatherittm and Proterotherium than 

 between the latter and Diadiaphortts. 



The scaphoid has proportions almost identical with those seen in Pro- 

 terotheriuni, but differs slightly in the shape of its facets ; the surface for 

 the trapezoid notches the bone more deeply, giving a more nearly cuboidal 

 shape to the distal extension which carries the magnum facet, while that 

 for the trapezium is somewhat smaller. — The lunar is proportionately not 

 quite so high and narrow as in Proterotherium and the proximal facet, for 

 the radius, does not incline so steeply toward the internal side, and on 

 the outer side of this facet is a very small, though distinctly marked, sur- 

 face for the ulna, a facet which does not appear in Proterotheriimi. The 



