Sinclair: typotheria of the santa cruz beds. 
77 
typotherium , with the inner lip exceedingly prominent. In fact the whole 
distal articular surface of the humerus is hardly to be distinguished from 
that of Protypotherium. The olecranon and coronoid fossae are separated 
by a very thin lamina of bone, which is sometimes perforated by a supra- 
trochlear foramen. An inner epicondylar foramen is always present. 
The head of the radius (PI. II, figs. 3, 4) is irregular in outline, much 
wider in transverse diameter than antero-posteriorly. The ulnar surface 
is almost plane transversely. The anterior margin supports a strong me- 
dian convexity, unlike the smooth, evenly convex, anterior margin of the 
radial head in Protypotherium. The outer two thirds of the head is cupped 
for articulation with the humeral capitellum. The inner third is concave 
antero-posteriorly, slightly convex transversely and slopes sharply inward. 
It articulates with the inner lip of the trochlea. The neck is transversely 
flattened, especially on the anterior side, where it is overhung by the head 
of the radius. The shaft is almost straight, while in Protypotherium it is 
strongly arched forward (cf. PI. IV, fig. 14). It is circular in cross-section 
proximally, but becomes triangular distally and at the extreme distal end 
irregularly quadrangular, owing to the development of grooves to accom- 
modate the extensor tendons. The carpal articular surface covers almost 
the entire distal end. The facets for the scaphoid and lunar are undiffer- 
entiated. The entire surface is deeply concave antero-posteriorly and 
more broadly concave transversely. The facet for the ulna is a small 
crescentic, antero-posteriorly concave surface on the ulnar side of the dis- 
tal end. 
The ulnar shaft (PI. II, figs. 6, 7) is much straighter than in Protypo- 
therium , exhibiting none of the lateral curvature characteristic of that 
genus. The olecranon process is proportionately about as long and wide 
as in Protypotherium — but is much narrower transversely. Anteriorly, it 
projects slightly beyond the coronoid process. The greater sigmoid cav- 
ity is narrower transversely than in Protypotherium. Its coronoid por- 
tion meets the lesser sigmoid cavity at a right angle, resembling closely 
the arrangement of these parts in many rodents. Both are approximately 
plane surfaces. The shaft is flattened laterally, lacking the strongly devel- 
oped interosseous ridge and the deep concavity on the inner face which 
are present in Protypotherium. Toward the distal end the shaft is ex- 
panded antero-externally, giving rise to a narrow, sharp ridge trending 
downward and outward. The inferior extremity of the ulna is heavier 
