M E U A L O N V X . 27 



process it expands iuto a large and rugged subeircuhir surface for muscular attach- 

 ment. Its base is pierced with a large but short oval canal, which commences at its 

 upper margin posteriorly, and proceeds obliquely to its anterior surface, and there 

 terminates in a deep concave fossa. This canal is a peculiarity in the anatomy of 

 the ller/ahnyx among bradypoid animals, as it does not exist in the recent sloths, 

 nor in Mi/Iodon, nor Mcgathermm. It is about two inches wide by one in height, and 

 is formed by a bridge of bone over an inch wide, which is extended from the shaft 

 near the end of the deltoid tract to the apex of the contiguous condyloid process. 



The outer condyloid process is a keel-like plate with a longitudinally convex 

 margin, which is rough and becomes thickened inferiorly. Between this process 

 and the boundary of the deltoid tract, the shaft of the bone presents a concave sur- 

 face descending from its outer to its anterior side, constituting the musculo-spiral 

 course, which does not appear so deep as in Mylodon, in consequence of the less 

 degree of prominence of the deltoid tract. 



The surface anteriorly of the humerus between the condyloid processes is tran.s- 

 versely slightly convex; but at the base of the inner condyloid process it presents 

 a deep concave fossa, previously mentioned; and just above the radial condyle it is 

 slightly and irregularly depressed. The posterior surface, between the condjdoid 

 processes, forms an extensive trilateral plane with the middle part slightly depressed, 

 but above the ulnar trochlea, it presents a shallow concavity to accommodate the 

 olecranon process. 



Inches. 

 Leugth of humerus ............ 20 



Breadth at tuberosities . . . . . . . . . . .5} 



Breadth at middle of shaft ........... 3^ 



Breadth at condyloid processes . . . . . . . . .10^ 



Breadth of cubito-brachial articulation . 5^ 



Circumference of shaft at middle ......... 9 



The adult specimen of a humerus, in the collection presented by Mr. J. P. 

 Wetherill to the Academy of Natural Sciences, and ascribed by Dr. Harlan to his 

 Megalonyx laqueaius,^ without doubt belongs to the same species as the preceding. 

 It is rather smaller than those above described, and the anterior surface between 

 the condyloid processes is more convex. In it also the concavity above the ulnar 

 trochlea posteriorly is deeper, and that above the radial condyle anteriorly, is more 

 distinct. Its length is eighteen and a half inches. 



Bones of the Forearm. — (PI. IX, Fig. 5 ; X, Fig. 1.) The bones of the forearm 

 of Megalonyx in the collection of Jefferson are nearly perfect. They are a third 

 longer than those of Mylodon robusius, but are narrower. 



Radius. — (PI, IX, Fig. 5; X, Fig. 1.) The radius is a long, clavate bone, slightly 

 bent forward ; and it is longitudinally convex upon its dorsal aspect and outer 

 margin, and concave upon the opposed surface and ulnar margin. The distal three- 

 fourths of the shaft ai'e about three times as broad as thick; and the upper fourth 



Journ. Ac. Nat. Sci., 1830, YI, ili, PI. 13, Fig. 10; Med. aud Phys. Res , 3-'l. 



