SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 37 



teriorly almost as much as internally. It varies considerably in size in 

 different individuals and, though never very large, it is much better devel- 

 oped and more conspicuous than in Macrauchenia. 



Proximally, the shaft is broad and strongly compressed and flattened 

 antero-posteriorly, narrowing and thickening downward and becoming 

 subcylindrical above the third trochanter, but with a strong forward cur- 

 vature. In the Pampean genus, on the other hand, the shaft has a greater 

 degree of antero-posterior flattening throughout. On the postero-external 

 side of the shaft and near the distal end (in Theosodon) is a large and 

 rugose pit for the attachment of the plantaris muscle and, on the anterior 

 face, there is a conspicuous suprapatellar fossa. The rotular trochlea is 

 narrow, deeply grooved and somewhat asymmetrical, due to its oblique 

 position and the greater elevation of its internal border ; distally, the 

 articular surface is continuous with that of the external condyle, but is 

 quite separate from the inner condyle. The condyles are rather small 

 and irregular and quite widely separated ; the external one is narrower 

 and more convex and projects more laterally, the internal one more 

 posteriorly. 



The patella (PI. XX, fig. 4) is a long, narrow, irregular and rather thin 

 bone, the antero-posterior thickness of which is much less than in Mac- 

 rauchenia. The proximal end is obliquely truncate, rising toward the 

 inner side, and the distal end narrows to a blunt point. The anterior face 

 is moderately convex and not very rugose, and the posterior, articular 

 face is divided by an obscurely marked ridge into narrower external and 

 wider internal portions. 



The leg-bones, which in Macrauchenia are coossified, are entirely sepa- 

 rate, and are considerably shorter than the thigh, thus reversing the pro- 

 portion of the segments which is found in the fore limb. 



The tibia (PI. XX, figs. 5, 5(3'), which is much shorter than the femur, 

 closely resembles that of Macrattchenia, except for its slenderness. The 

 proximal end is very heavy ; the external condyle is narrower antero- 

 posteriorly, but more concave than the internal, which is nearly plane. 

 The spine is deeply bifid, and the portion formed by the external condyle 

 is higher, broader and heavier than that which belongs to the inner con- 

 dyle. The cnemial crest is short, soon dying away upon the shaft, but 

 its proximal portion is very massive and rugose and is deeply impressed 

 by the fossa for the insertion of the great extensor tendon of the thigh. 



