SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. I3I 



Above the distal end, the shaft is abruptly contracted, to receive the distal 

 expansion of the radius, and there is a rather large, convex facet for the 

 latter. The surface for the pyramidal is saddle-shaped and has its prin- 

 cipal diameter in the dorso-palmar dimension ; it is continuous with the 

 small facet for the pisiform, which is quite internal in position. 



In Macratcchenia, the fore-arm bones are coossified, though not always 

 completely so ; they are broader transversely and more compressed antero- 

 posteriorly than in Theosodon, even at the distal end. Proximally, the 

 ulna is much heavier than the radius and the relative proportions of the 

 two bones remain very much as they are in the Santa Cruz genus. 



The manus of Theosodon (PL XIX, fig. 6) is surprisingly like that of 

 Macrattchenia even in the minor details, as has already been pointed out 

 by Ameghino ('94<5, 281). However, a few slight, though not entirely 

 insignificant, changes may be observed in the Pampean genus. The 

 manus is of mesaxonic symmetry and is tridactyl, not only structurally, 

 but functionally, the digits being of nearly equal length and thickness, 

 and each carrying its due proportion of the weight. 



The carpus (PI. XIX, figs. 6, da) is low proximo-distally and wide 

 transversely, though relatively somewhat higher and narrower than in 

 Macrauchenia. Lydekker's statement ('96, 79) that in the Pampean genus 

 the arrangement of the carpal elements is in vertical series, without inter- 

 locking, is not quite accurate. As will be seen, there is a certain degree 

 of interlocking, even in the Santa Cruz genus, and this is increased in 

 Macrmtcheiiia. 



The scaphoid is low, broad and very thick palmo-dorsally, and of irreg- 

 ularly quadrate outline ; the proximal end is covered by the radial facet, 

 which lias a dorsal convexity, dying away upon the inner side, and a 

 palmar concavity. On the distal end are three facets ; internally, a small 

 concavity for the trapezium, then follows a very large surface for the 

 trapezoid, composed of dorsal and palmar concavities, the junction of which 

 forms a ridge. Finally, there is a heavy distal process, which projects some- 

 what internally and carries a facet for the magnum and, on the ulnar side, 

 one for the lunar. This extension of the scaphoid over upon the magnum 

 is the only departure from the serial arrangement to be found in this 

 carpus. 



The lunar is very narrow, but moderately high proximo-distally, and 

 very thick palmo-dorsally. The radial surface is strongly convex, extend- 



