SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 29 



ward, but not at all distally, the distal border of the spine being nearly- 

 straight and normal to the plane of the blade. 



The fore-limb is long and slender and its three divisions are propor- 

 tioned somewhat as in Hyracodon, the cursorial rhinoceros from the 

 Oligocene of North America. That is to say, the upper arm is very short, 

 the fore-arm is greatly, and the manus moderately elongate. 



As just indicated, the humerus (PI. XVIII, fig. lo ; PI. XIX, fig. 5) 

 is relatively short and stout, hardly more than two-thirds the length of the 

 skull. The head is large and rather flattened and projects backward but 

 little ; the external tuberosity is very largely developed, as a high, massive 

 ridge, which rises far above the level of the head, while the internal tuber- 

 osity is very much smaller and less prominent. The bicipital groove, 

 which is single and without any tubercle, is wide and deep ; the deltoid 

 crest is prominent and rugose and extends down for nearly half the length 

 of the shaft, ending abruptly below. The shaft is proportionately quite 

 massive and of the usual ungulate form. Proximally, the shaft is much 

 compressed laterally and very thick'antero-posteriorly, though this latter 

 dimension is not so exaggerated as in Macrauchenia. Distally, the shaft 

 becomes wider transversely and thinner antero-posteriorly ; the epicon- 

 dyles are massive and the external one is quite prominent ; there is no 

 entepicondylar foramen. The supratrochlear fossa is well-marked and 

 the anconeal fossa is large, of irregular shape and deep, but the two fossae 

 are not connected by a perforation. The trochlea, which is placed some- 

 what obliquely to the long axis of the shaft, rising toward the inner side, 

 is extremely simple and forms a pulley-shaped surface not unlike the 

 astragalar trochlea of a rhinoceros. Of the two borders, the internal is 

 the sharper and more elevated ; posteriorly, the groove for articulation 

 with the ulna is very deep. The articular surface is continuous through- 

 out and shows no indication of a division into facets and there is no 

 median trochlear ridge. 



The bones of the fore-arm (PI. XIX, fig. 2) are long and slender and, 

 though without any tendency to coossification, they are in close contact 

 throughout their length and there is no radio-cubital arcade. Proximally, 

 the ulna is much stouter than the radius ; distally, the radius is the heavier 

 of the two. 



The radius (PI. XIX, figs. 2, 3, 3«) is elongate and very slender. 

 Indeed, in view of the tridactyl and mesaxonic manus, the enlargement of 



