414 , ANATOIMY OF VERTEBRATES. 



sight into the mode of life of extinct animals, in a striking degree 

 through its application to the skeletons of the Megatherioids.^ The 

 teeth of these conform so closely in all characters with those of 

 the Sloths as to suggest leaves rather than roots to have been 

 their food. In the light slender Sloths the modifications of struc- 

 ture for clunbing, clinging, and living altogether in trees, are 

 carried out to an extreme. In the colossal extinct kinds the 

 foliage was obtained in a different way. The huge single claw 

 on the hind foot would be applicable as a pickaxe to clear away 

 the soil from between the ramifications of the roots : a second 

 claw would have interfered with such work. The foot is organised 

 to 2;ive oreat strength to that claw ; dislocation of its toe is 

 specially guarded against: the rest of the tarso-metatarsal structure 

 relates to the power of the foot to sustain superincumbent pres- 

 sure, with a position of the claw bringing its side instead of its point 

 in contact with the ground. The bones of the thigh and leg are 

 remarkable for their massive proportions, for their thickness, and 

 especially their breadth in proportion to their length : the femur 

 in both Mijlodon and Megatherium would rank rather with the 

 'flat ' than the ' long ' bones. These osseous columns were needed 

 to support the huge, heavy, expanded pelvis, fig. 267. The iliac 

 expansions are the chief conditions of the other characteristics of 

 this part : and they are unintelligible save in relation to adequate 

 extent of origin of povverful muscles, especially those arising 

 from the crista ilii, 62, the chief of which muscles concentrate 

 their force upon the fore-limbs. This indicates that these limbs 

 were put to some unusual work ; and the inferences from the 

 teeth and the hind claw lead to its recognition as the pulling 

 down trees and wrenching off their branches : but, for these 

 operations, the pelvis must have adequate fixity ; and to the 

 weight and strength of itself and its supporting limbs there is 

 added a tail so developed as to serve as a third support and 

 give the pelvis the basis of a tripod. Without this view of the 

 function of the hind-parts of the skeleton we can only see that 

 the peh is is so great and, with its caudal appendage, so weighty 

 as to require the massive proportions and structure of the hind- 

 limbs ; and, reciprocally, that these bespeak a proportionate size 

 and weight of the parts to be sustained : but why such develope- 

 ment of sustaining limbs and parts to be supported in reference 

 to any other action and way of life is inconceivable. The excess 

 of bone in the hind-part of the skeleton once recognised as relating 

 to the fixed point of attachment of muscular forces working the 

 fore-limbs, — to the exertion of poAver adequate to prostrate a 



