30 SCOTT. [Vol. V. 



mens it is considerably broader than in others), and the spine 

 rises nearer to the coracoid than to the glenoid border, making 

 the prescapular fossa much broader than the postscapular, but 

 the difference is not so great as in Auchenia, and the outline 

 formed by the coracoid border is rather more like what we find 

 in the camel than in the llama. The spine rises abruptly to a 

 considerable height and runs in a somewhat straighter and less 

 sinuous line than in either of the recent genera ; the acromion 

 is very long, expanded at the free end, and somewhat retro- 

 verted. The glenoid cavity is shallow and nearly circular in 

 outline, the antero-posterior diameter but slightly exceeding the 

 transverse ; this varies, however, in different specimens, as does 

 also the degree of thickening and rugosity shown by the edge 

 of the glenoid cavity. The coracoid process is less prominent 

 than in the existing genera and very much less massive and 

 rugose ; as in them, however, it is curved inward at the free 

 extremity. 



The humerus (PI. II, Figs. 26-28) is strikingly different from 

 that of the modern representatives of the group, and in the 

 same way as the humerus of Mesohippus differs from that of 

 the existing horses. In Pcebrothcrium the humerus is relatively 

 long and slender, with feebly developed ridges for muscular 

 attachments ; the deltoid ridge and hook, so conspicuous in the 

 recent forms, are here but a roughened line ; and the supinator 

 ridge, though running far up upon the shaft, is but faintly indi- 

 cated. The shaft shows the same sigmoidal curve as in Came- 

 lus and Auchenia, but to a less degree, and is very much more 

 slender, especially when seen from the front, for the antero- 

 posterior extent of the proximal part of the shaft is considerable. 

 The head is strongly convex, is more sharply constricted off 

 from the shaft, and presents more decidedly backward than in 

 the recent genera. The external tuberosity is a high and 

 roughened ridge, which extends across nearly the entire anterior 

 face of the bone, and ending in a prolonged and massive hook 

 which overhangs the bicipital groove. The upper margin of 

 this tuberosity is slanting on the external side, projecting but 

 little above the head, but rising high above it on the internal 

 side. The internal tuberosity is small and is separated from 

 the outer one only by the narrow and deep bicipital groove, the 

 bottom of which is occupied by the very small beginning of 



