PROFESSOR OWEN ON THE EOSSIL MAMMALS OF AUSTRALIA. 553 



being turned so as to form rather the outer facet or side of the upper half of the humeral 

 shaft. Another peculiarity of the present humerus is a well-defined oval rough surface 

 (fig. 1, h) at the outer side of the back part of the shaft, one-fourth of the way down ; 

 this surface measures 2 inches 2 lines lengthwise, by 1 inch 2 lines across ; the lower 

 half of its periphery is most prominent. A low ridge, about an inch in length, in Macro- 

 ipus major, seems to answer to this process. 



The shaft expands transversely and becomes flattened from before backward at its 

 lower third as far as the distal articulation (fig. 4), which resumes antero-posterior thick- 

 ness with reduction of transverse extent. The ento-condylar ridge [i) is much produced, 

 though relatively less than in Macropus ; the upper and lower borders meet at an open 

 angle ; the ridge is very thick ; it extends more than 2 inches from the ulnar condyle ; 

 it is imperforate. 



The ectocondylar ridge {k) is longer than the inner one (i), but is less prominent ; it 

 is also angular in form, but more openly so. The upper and longer side, commencing 

 6 inches above the radial condyle, is narrow and slightly turned forward ; the lower side 

 rapidly expands to nearly the fore-and-aft breadth of the radial condyle, along the radial 

 or outer border of which the ectocondylar ridge subsides. The middle of the distal 

 expansion of the shaft, above the articulation, is remarkable for the size and depth of 

 the anterior fossa (fig. 2, 1) ; the posterior or olecranal depression (fig. 1, m) is compa- 

 ratively feebly marked. 



The radial (w) and ulnar (o) condyles are more convex and more equal than in Macro- 

 jpus ; they are divided by a narrower and deeper trochlear channel. 



The radial condyle is the longest ; its outer and hinder marginal contour describes 

 part of a circle, and is not encroached upon, as in Macropiis, by the rough surface from 

 the ectocondylar tuberosity. The ulnar condyle, which begins in Macropus to subside or 

 give way to augment the intermediate concavity, here retains its hemispheroid form 

 (ib. fig. 4, o). It is interesting to note this resemblance to the distal articulation of a. 

 femur in the humerus of an animal low in the Mammalian scale. 



The hind or " olecranal," supracondylar depression (fig. 1, m) is shallow, limited in 

 situation to above the ulnar condyle, and the narrow intercondylar channel, and not ex- 

 tending to above the radial condyle. 



I have not been able to find the orifice of a medullary artery : the distal portion of 

 the left humerus, broken from the rest of the bone near the middle of the shaft, 11^ 

 inches from the distal end, shows a depression near the middle of the fractured surface 

 half an inch across and not quite an inch deep ; and this, if it be not an accidental 

 excavation in the dense cancellous structure, is the sole indication of a medullary cavity. 

 Such cavity is wanting or small in the gigantic extinct Sloths. It is, again, with interest 

 that I view this sign of low organization in the great extinct Marsupial mammal *. 



Ulna. — Of the ulna I have hitherto received only the proximal half, and with the 



* Description of the Skeleton of Mylodon rohustus, 4to, 1842, p. 82. Memoir on the Megatherium, 4to, 

 1860, p. 49, 



4f2 



