SINCLAIR: TYPOTHERIA OF THE SANTA CRUZ BEDS. 
27 
tally it becomes wedge-shaped, the point of the wedge corresponding to 
the sharp interosseous margin, while the head of the wedge forms the 
convex anterior surface of the shaft. The head is oval, narrower inter- 
nally than externally and slightly cupped for reception of the humeral 
capitellum. The inner narrower portion of the oval is occupied by a 
surface inclined downward and inward, slightly convex transversely and 
articulating with the inner portion of the humeral trochlea. A slight 
projection on the anterior margin of the radial head separates the margins 
of these two surfaces. The ulnar surface is slightly convex transversely 
and flattened vertically. The carpal articular surface is small in propor- 
tion to the width of the shaft at this extremity. It is irregularly oval in 
outline, concave in all dimensions and shows faint traces of division 
into two surfaces for the scaphoid and lunar respectively (PI. IV, fig. 
13.) The styloid process is quite short and inconspicuous. 
The ulnar shaft (Pis. IV, figs. 16, 17; VI, fig. 3) is strongly curved 
laterally. The interosseous border is almost straight and strongly rugose, 
and the posterior border smooth and broadly sigmoid in outline. On the 
outer surface the shaft is excavated longitudinally for about half its length 
below the sigmoid cavity. The olecranon is heavy, posteriorly flattened 
and strongly rugose distally and projects as far forward as the coronoid 
process. The latter is quite sharp, the humeral and radial articular sur- 
faces meeting at an acute angle, as in many of the rodents. Distally, the 
anterior margin of the shaft curves outward, leaving a flattened area above 
the radial facet (PI. IV, fig. 16). The latter is convex from side to side 
and plane proximo-distally. In contrast with Hegetotherium , the styloid 
process is long, narrow and more sharply pointed. 
The manus is tetradactyl, with interlocking carpus and metacarpus (PI. 
V, figs. 3, 4, 9 ; text fig. 3, A). The second and. third digits are of approxi- 
mately the same size, the third slightly exceeding the second in length, 
with the axis of the foot passing between them. The fourth digit is much 
shorter and feebler and the fifth greatly reduced. No trace of the first 
remains. Proximally, the scaphoid presents a broad, antero-posteriorly 
convex surface for articulation with the radius. Distally, it is in contact 
with the magnum by a long, narrow process (scaphoid + centrale) support- 
ing a small, plane, quadrangular facet, and with the trapezoid by a large 
crescentic facet, plane in dorso-palmar section and slightly concave trans- 
versely. Externally, it articulates with the lunar by a small, oval, proximo- 
