FOSSIL MAMMALIA. 



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ordinary Ruminants they are partially joined by bony confluence, which rarely 

 extends to the proximal extremities ; in the Camel and Llama the anchylosis of 

 the radius and ulna is so complete, that no trace of their original separation can 

 be perceived, and the olecranon appears but as a mere process of the radius. 



In the Macrauchenia the anchylosis of the radius and ulna is also complete, 

 but the boundary line of the two originally distinct bones is very manifest, and the 

 proportion which each contributes to the great articulating surface for the distal 

 end of the humerus is readily distinguishable. About a sixth part of this surface 

 is due to the head of the radius, which enters into the composition of the anterior 

 and outer part of the articulation, and its extent is defined by a depressed line 

 describing a pretty regular curve, with the concavity directed forwards and a 

 little outwards, (a, fig. 1, PI. X.) Just below the articular surface a strong 

 triangular rugged protuberance projects from the front of the head of the radius, 

 for the attachment of the tendon of the biceps. The line of separation of the radius 

 and ulna is indicated on the inner side of the head of the radius by a deep and 

 narrow fissure extending downwards from below the anterior part of the articula- 

 ting surface ; and on the outer side by a broad groove leading upwards to a deep 

 pit near the proximal end of the antibrachium. We may see by the direction of 

 the head of the radius which is thus defined, that it crosses obliquely in front of 

 the ulna, as in the Elephant, Hippopotamus, and other Pachyderms, and that 

 the bones are anchylosed in the prone condition : below this fissure and groove, 

 which mark the interosseous line, the radius and ulna become blended together 

 into one compact bone, which is flattened from before backwards, with a well 

 marked ridge on the outer side; and excavated by a single medullary cavity, 

 the compact walls of which present a general thickness of one-third of an inch. 



The proximal articular surface or sigmoid cavity of the antibrachium, con- 

 stituted as above described, resembles that of the Palseothere, Tapir, and the 

 generality of the Pachyderms in having two depressions, instead of three, as in 

 the Anoplothere, and Ruminants. The Hippopotamus has a slight tendency to 

 the latter structure, which is also less marked in the Camel than in the ordinary 

 Ruminants. In its general form the sigmoid cavity of the Macrauchene resem- 

 bles that of the Hippopotamus more than that of the Camel. In the Camel this 

 articular surface is traversed transversely by a broad, shallow, and slightly 

 roughened tract, which divides the smooth surface of the joint into two parts, 

 one forming the anterior horizontal surface due to the conjoined radius and ulna, 

 the other forming the vertical concave surface on the anterior part of the base 

 of the olecranon. In the Hippopotamus there is, as it were, an attempt at a 

 similar division of the articulating surface at the proximal end of the antibra- 

 chial bones ; a deeper and rougher depression encroaches upon the articulation 

 from its outer side, but stops when it has reached half-way across. In the 



