SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 25 



than in that phylum : the proximal end is divided unequally between 

 the scaphoid and lunar facets, the latter being somewhat the smaller of 

 the two : on the radial side are two nearly equal facets for the trapezoid 

 and mc. II ; the distal facet for mc. Ill is convex and oblique, rising toward 

 the ulnar side, on which is a relatively large, concave surface for the pyram- 

 idal : there is no articulation with the unciform. 



The unciform, though considerably larger than the trapezoid, is yet much 

 smaller than the magnum : it is narrow, thick and moderately elongate 

 proximo-distally : in correlation with the great length of the pyramidal, 

 the unciform projects much farther distally than any of the other carpals, 

 its proximal surface being below the distal end of the trapezoid. Conse- 

 quently, as already noted, there is no contact between the magnum and 

 unciform, the radial side of the latter being covered by the process of mc. 

 Ill : on the distal end is a concave facet for the head of mc. IV and, palmar 

 to this, a small surface for the rudimentary mc. V. 



The metacarpus is tridactyl and mesaxonic and, though the lateral 

 digits have their full complement of phalanges, they are mere dew-claws and 

 can have had little or no functional importance. Ameghino states ('94/^, 

 p. 268) that in this genus there is a vestigial mc. I, which is quite long and 

 extremely slender and is placed on the palmar side of the manus. I have 

 seen no example of this. 



Metacarpal II is much shorter and very much more slender than mc. 

 Ill ; the proximal end is relatively heavy and quite extensively overlaps 

 the head of mc. Ill and abuts against the radial side of the magnum, thus 

 entirely excluding mc. Ill from any contact with the trapezoid. The shaft 

 is slender, much compressed laterally, but of considerable dorso-palmar 

 diameter, and has a strong forward curvature ; the distal trochlea is very 

 narrow, but has a well-defined carina, which is reflected over upon the 

 dorsal face of the trochlea. 



Metacarpal III is very stout and moderately elongate ; the proximal end 

 is supported only by the magnum, for though there is quite an extensive 

 articulation with the unciform, this contact is entirely lateral. On the 

 other hand, the overlapping head of mc. II, by means of which part of the 

 weight is transmitted from the radius through the scaphoid and trapezoid 

 to mc. Ill, may be regarded as functionally equivalent to another carpal 

 element. On the ulnar side, mc. Ill hardly projects at all over the head 

 of mc. IV. The shaft is heavy, palmo-dorsally compressed and broad 



