EDENTATA OF THE SANTA CRUZ BEDS. 343 



bluntly conical tubercle and not a roughened ridge ; the third trochanter 

 is much less prominent than in the Santa Cruz Megalonychidce and of 

 quite a different shape ; it does not, as in Hapalops, rise abruptly from 

 the shaft, but forms a long, gentle convexity, the middle part of which is 

 thickened and rugose and represents the trochanter proper, much as in 

 Megalonyx, though larger. The shaft is very broad and antero-posteriorly 

 compressed ; the anterior face is convex transversely, slightly concave 

 longitudinally, and its proximal half has parallel ridges for muscular 

 attachment, as in the Pleistocene genera ; on the hinder face a low, but 

 distinct, ridge runs obliquely from the great trochanter to the inner side 

 of the shaft, somewhat below the middle. The external tuberosity is 

 very prominent, more so than in Megalonyx, but less than in Megatherium, 

 while the internal is even larger than in the latter. 



The distal articular surfaces have a significant likeness to those of Meg- 

 atherhtm, though they are much less specialized ; the condyles are of more 

 nearly equal size and not so widely separated ; the external condyle is less 

 reduced, especially in width ; the rotular trochlea is broader and less con- 

 cave transversely, and is connected with both condyles, but in a very sig- 

 nificant way. In Megalonyx the external condyle is separated from the 

 trochlea and the inner condyle is connected with it " only by a narrow, 

 isthmus-like prolongation " (Leidy, '55, 38) but the articular surfaces are 

 not continuous, which led Owen to suggest that in this genus the knee- 

 joint had three synovial capsules ('60, 813). In Hapalops the articular 

 surfaces of both condyles are continuous with the trochlea, but the Mega- 

 lonyx type is present in an incipient degree, the connection being much 

 narrowed by an extension of the intercondylar notch on both sides. In 

 Megatherium the external condyle is broadly continuous with the trochlea, 

 but the internal condyle is widely separated from the latter. As regards 

 the distal end of the femur, Prepotherimn stands in the same relation to 

 Megatherium as Hapalops does to Megalonyx ; both condyles are con- 

 nected with the trochlea, but, while the external one is broadly so con- 

 nected, the internal is united with the trochlea by a very narrow bridge. 



The tibia (Plate LXI, figs. 2, 2") is not very different from that of Hapa- 

 lops, but its differences are in the direction of Megatherium. The proximal 

 end is broad, very heavy and somewhat compressed antero-posteriorly; 

 the condyles are more equal than in Hapalops and the spine is less promi- 

 nent ; the cnemial process is a massive transverse ridge, which is not ex- 



