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92 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 





ness : but in all these circumstances the affinity of the Scelidothere with the 

 Megathere is prominently brought into view. There is no other known quadruped 

 with which the Scelidothere so closely corresponds in this respect. In proceeding, 

 however, to compare together the thigh-bones of these two extinct quadrupeds, 

 several differences present themselves, which are worthy of notice : of these the 

 first is the presence in the Scelidothere of a depression for a 'ligamentum teres' on 

 the back part of the head of the femur, near its junction with the neck of the bone : 

 this is shewn in the posterior view of the femur given in PI. XX. The head itself 

 forms a pretty regular hemisphere : the great trochanter does not rise so high as 

 in the Megatherium, but, relatively, it emulates it in breadth: the small trochanter 

 is proportionally more developed : the external contour of the shaft of the femur 

 is straighter in the Scelidothere than in the Megathere, and the shaft itself is less 

 bowed forwards at that part. The articular condyles occupy a relatively smaller 

 space upon the distal extremity of the femur in the Scelidothere, and they differ 

 more strikingly from those of the Megathere, in being continued one into the 

 other : the rotular surface, for example, which is shewn in fig. 5. PI. XXV. is 

 formed by both condyles, while in the Megatherium it is a continuation exclusively 



of the external articular surface. 



The patella, which works upon the above-mentioned surface, is a thick strong 

 ovate bone, with the smaller end downwards : rough and convex externally, 

 smooth on the internal surface, which is concave in the vertical and convex in the 



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transverse directions. 



Of the bones of the leg only the proximal end of the tibia is preserved ; but 

 this is valuable, as shewing another well-marked difference between the Scelido- 

 there and Megathere ; for whereas in the latter the fibula is anchylosed with the 

 tibia, this bone, in the Scelidothere, presents a smooth flat oval articular surface, 

 which is shewn in fig. 2. PI. XXVII. below the outer part of the head of the bone ; 

 from the size and appearance of which, I infer, that the fibula would not have 

 become confluent with the tibia, even in the mature and full-grown animal. 



The relative length of the fore and hind extremities cannot be precisely deter- 

 mined from the present imperfect skeleton of the Scelidothere ; but there is good 

 evidence for believing, that the fore extremity was the shortest. The humerus is 

 shorter than the femur by one-ninth part of the latter bone ; and the radius, 

 which wants only the distal epiphysis, must have been shorter than the hu- 

 merus. Now the relative development of the fore and hind legs is one of 

 the points to be taken into consideration in an attempt to determine the habits 



and nature of an extinct mammal. 



In climbing animals the prehensile power is more essential to the hinder than 

 to the fore parts or extremities. In the leech the principal sucker is in the tad ; 



