SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 33 



The magnum is stout, but not very large, though it has a complete 

 system of articulations, being in contact with the second and third meta- 

 carpals and all of the carpals except the trapezium and pisiform. The 

 proximal end is unequally divided between the facets for the scaphoid and 

 lunar, the latter being somewhat wider and more completely proximal, 

 while the former has an oblique position. The "head " of the magnum 

 is quite low and there is no hook given off from the palmar face. On the 

 ulnar side are two nearly plane facets, a proximal one for the pyramidal 

 and a distal one for the unciform. Distally, the entire surface of the 

 magnum is covered by the large, somewhat saddle-shaped facet for the 

 third metacarpal, and on the radial side, near the distal end, is an oblique 

 facet for the second metacarpal, which extends across the whole dorso- 

 palmar diameter of the magnum. Above this is a small surface for articu- 

 lation with the trapezoid, and this facet is confined to the dorsal moiety 

 of the magnum. 



In Macrauchenia, as above indicated, the magnum is further modified 

 by its extension under the pyramidal, or the overlapping of the pyramidal 

 upon it. In this way the facet on the magnum for the pyramidal becomes 

 proximal and an interlocking of a very exceptional type is brought about, 

 the magnum lying beneath the scaphoid on one side and beneath the 

 pyramidal on the other, while the lunar is excluded from any contact with 

 the unciform. In all Perissodactyla, on the contrary, interlocking is pro- 

 duced by the growth toward the ulnar side of the scaphoid and lunar, 

 accompanied by the reduction of the pyramidal. Thus the scaphoid rests 

 more or less extensively upon the magnum and the lunar on the unciform, 

 the pyramidal being excluded from the magnum. In the Proboscidea, as 

 in the Macrauchenidse, the magnum articulates with both scaphoid and 

 pyramidal, and, in both groups, this exceptional arrangement is, no doubt, 

 due to the relatively large size of the ulna. 



The unciform of Theosodon is considerably larger than the magnum ; 

 it is low, broad and thick and has a short, massive and rugose process 

 projecting from the palmar face. On the proximal end is a large, saddle- 

 shaped surface for the pyramidal, and on the distal end, an almost equally 

 large facet for the fourth metacarpal, which, however, narrows much 

 toward the palmar side. A very small distal facet indicates the presence 

 of the vestigial fifth metacarpal. On the radial side are two facets, a large 

 one for the magnum and a smaller, oblique one for the third metacarpal. 



