go FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



and in its prominent flat-faced deltoid ridge. This occupies about three- 

 eighths of the inner proximal part of the front of the humerus, and instead 

 of forming a single trenchant ridge it presents an elevated flat rhom- 

 boidal surface extending downward from the greater tuberosity and 

 ending bluntly and abruptly. Its surface is slightly roughened and lies 

 at a considerable angle to the long axis of the humerus. From its distal 

 extremity the front of the shaft abruptly recedes and forms a long 

 sweeping curve to the condyles. The deltoid ridge or tuberosity has a 

 flattened surface in some other forms, as Perameles, Dasyurus, and 

 Peramys, but in these it is triangular, coming to a point distally. In 

 the fossil form Garzonia patagonica, of which a humerus is figured by 

 Sinclair (1906, PI. XL/Ill, fig. n), the deltoid surface is very broad, 

 and although it is less blunt distally, it seems to approach the condition 

 in Ccsnolestes more nearly than does that of any recent form. 



The head of the humerus is rather large and has considerable back- 

 ward extension beyond the axis of the shaft. The external or so-called 

 greater tuberosity is much smaller and less marked than the internal. 

 It is greatly compressed and its outer surface is continuous with the 

 deltoid ridge. Its upper surface is continuous medially with the surface 

 of the head of the humerus. The internal tuberosity is prominent and 

 distinctly separated from the head by a deep groove as well as from the 

 external tuberosity by the bicipital groove. A well-marked ridge ex- 

 tends distad from it parallel with the internal border of the deltoid 

 ridge and terminating in a roughened area for the insertion of the 

 latissimus muscle. 



The middle of the shaft of the humerus for about one-third the length 

 of the bone is relatively smooth, slightly compressed, subcylindrical, 

 with only traces distally of the upper continuation of the external 

 epicondylar or supinator ridge. The caudal outline from the head of 

 the humerus to the supinator ridge is a deep sigmoid curve. The distal 

 end of the humerus is broadly and rather abruptly expanded. The 

 internal condyle is prominent and the sulcus between it and the ulnar 

 trochlea is developed as an articular surface for the inner upper lip of 

 the sigmoid cavity of the very obliquely situated ulna. The trochlea 

 is somewhat smaller than the capitellum with which it is continuous. 

 Thus there are three articular surfaces at the distal end of the humerus, 

 a very unusual condition. The third or internal articular surface is fully 

 developed only on the caudal aspect of the bone and does not continue 

 to the front as the trochlea and capitellum do. Although partly con- 

 fluent, these three articular surfaces are sufficiently distinct from each 

 other, but of course the third or inner one could be regarded as an 

 extension of the trochlea which might be developed without difficulty. 



