No. i.J OSTEOLOGY OF PCEBROIHERIUM. 31 



what Kowalevsky has named the "bicipital tubercle" (No. 16, 

 p. 6). The bicipital groove is thus confined to the antero- 

 internal angle of the head, and is overhung by the external 

 tuberosity so as to be hardly visible at all when viewed from 

 the front. At first sight, therefore, the proximal end of the hu- 

 merus of Poobrotherium appears to be totally different from that 

 of the modern Tylopoda, in which the external tuberosity is but 

 little larger than the internal, from which it is separated by a 

 very wide interval : the bicipital groove has immensely broad- 

 ened, and is separated into two parts by the bicipital tubercle, 

 which has become almost as large as the tuberosities proper, 

 and is situated in the median line of the humerus. Great as 

 this difference is, it is entirely bridged over, as we shall see 

 later, by the intermediate genera. 



The distal end of the humerus is proportionally narrow ; the 

 trochlea is set somewhat more obliquely to the long axis of the 

 shaft than is the case in the modern forms, and has a consider- 

 ably greater height on the internal than on the external side — a 

 difference which is hardly indicated in the camel and llama. The 

 intercondylar ridge is a faintly marked, low, narrow, and rounded 

 swelling, and differs strikingly from the broad and prominent 

 ridge of Camelns and Auckcnia. The external epicondyle is but 

 little developed, forming merely a stout ridge, but the internal 

 one is quite large and forms a prominent rugose projection at 

 the postero-internal angle, very much as in the camel : in the 

 llama this is considerably reduced. The trochlea is higher than 

 in the recent species, and especially is carried up on the poste- 

 rior side of the bone very much higher than in the camel — a 

 mode of development which reverses that which occurs in the 

 true ruminants. The anconeal fossa is small but deep and per- 

 forates the shaft, thus forming a small supratrochlear foramen. 



The ulna and radius (PI. II, Figs. 29-32) are firmly co-ossified 

 throughout their length, and are remarkably long and slender, 

 though not quite so long as in the existing Tylopoda. Assum- 

 ing the length of the humerus to be 100, that of the radius is 

 115, while in the camel it is 125. Just below the proximal end 

 there is a small interosseous foramen which penetrates from 

 side to side, and above the distal end there appears to be 

 another ; at all events there is a very deep anterior fossa 

 between the two bones at this point. 



