^THE OSTEOLOGY OF ELOTHERIUM-. 299 



The humerus is relatively long, but is, at the same time, a massively constructed 

 bone. The head is large and very strongly convex, especially from above downward, 

 although it is not set upon a very distinct neck, nor does it project far behind the plane 

 of the shaft. The external tuberosity is very large, forming a massive and roughened 

 ridge, which runs across the whole anterior face of the head and rises toward the internal 

 side, where it terminates in a high, thick and recurved hook, overhanging the bicipital 

 groove. The internal tuberosity is very much smaller, but is, nevertheless, quite promi- 

 nent ; it likewise projects over the bicipital groove, which is very broad and deeply 

 incised into the bone. The great transverse breadth of the external tuberosity displaces 

 the groove far toward the internal side of the humerus. The shaft is long and heavy ; 

 its proximal portion has a great antero-posterior diameter, and its transverse thickness, 

 though less, is still very considerable. The fore-and-aft diameter gradually diminishes 

 downward, until the shaft assumes an almost cylindrical shape, below which point it 

 begins to expand transversely. The deltoid ridge is rugose and prominent, and runs far 

 down upon the shaft, but forms no deltoid hook. The distal end of the shaft is very 

 heavy, being both broad and thick. The supratrochlear fossa is low, wide and shallow, 

 while the ancoueal fossa is very high, narrow and deep, its depth being much increased 

 by the great production of the posterior angles of the distal end. The supinator ridge is 

 rough, heavy and prominent. The trochlea, which is very completely modernized, in 

 correspondence with the advanced differentiation of the ulna and radius, is somewhat 

 obliquely placed with reference to the long axis of the shaft, descending toward the ulnar 

 side. The trochlea differs very markedly from that of such primitive artiodactyls as 

 Oreodon and Anoplotherium ; it is high, full and rounded and is divided into two unequal 

 radial facets, of which the inner one is decidedly the larger. The intercondylar ridge, 

 which, in most primitive artiodactyls, forms a broad and rounded protuberance, is, in 

 Elotherium, compressed into a sharp and prominent ridge, and shifted well toward the 

 external side. The internal epicondyle, which is so largely developed in Oreodon and 

 other early artiodactyls, has practically disappeared. 



The humerus of Hippopotamus is relatively much shorter and more massive than 

 that of Elotherium ; the external tuberosity is not extended so far across the anterior 

 face of the bone and the bicipital groove is, in consequence, not shifted so far toward the 

 inner side ; the deltoid ridge is much better developed and gives rise to a prominent 

 deltoid hook. In the existing species of Hippopotamus the intercondylar ridge is 

 narrower and less conspicuous, but in a Pliocene species from the Yal d'Arno it has 

 quite the same appearance as in Elotherium (see de Blainville, Osteographie, Hippopot- 

 amus, PL V). The epicondyles are much more prominent than in the latter, and 

 the postero-internal border of the aneoneal fossa projects much more than does the 



