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langes of the Armadillos, which offer in other respects the nearest resemblance 

 to those of the Mylodon, differ by their obliquity more than do those of the Ant- 

 eaters and Sloths. In the large proportional size of the middle digit the hand of 

 the Dasypus gigas offers only a repetition of the secondary feature of resemblance, 

 which is better marked and is associated with fewer deviations from the Mylo- 

 dontal type in the Ant-eaters. 



If now, having searched in vain among the pentadactyle Edentata for a repeti- 

 tion of the most characteristic and essential modification of the carpus of the 

 Mylodon, we find this very modification present in that of the Sloths, we cannot 

 avoid acknowledging the essential affinities of these remarkable quadrupeds to 

 the extinct Megatherioids. Even in the didactyle Unau, in which the adaptive 

 modifications of the Megatherioid type of manual organization are carried to an 

 extreme in relation to the exclusively arboreal hfe of the species, the carpus con- 

 sists of seven bones, as in the Mylodon, and the diminution of the normal num- 

 ber results from the same coalescence of the scaphoid and trapezium, This 

 structure, which Cuvier was the first to recognize in the recent Sloths, he con- 

 tinued to afiirm in the latest edition of the ' Ossemens Fossiles,' to be wholly pe- 

 culiar to them. The manus of the Megatherium he regarded as most resembling 

 that of the Dasypus gigas, and the able Editor of the posthumous edition, al- 

 though inferring, from a cast of that part of the Megatherium in the Royal Col- 

 lege of Surgeons, that it had a greater analogy with the manus of the Myrmeco- 

 phagajubata, yet seems not to have detected, what in the original is plainly evi- 

 dent, the repetition of the anchylosis of the scaphoid and trapezium in the carpus 

 of the Megatherium. In no existing quadrupeds, save the Sloths, and in no 

 extinct species, except the Mylodon and Megatherium, has this peculiar modi- 

 fication of the carpus been observed. The form of the scapho-trapezial bone 

 in the Unau corresponds also pretty closely with that in the Mylodon : it presents 

 a smooth convexity to the radius, parallel with that of the os lunare, and both 

 sloping towards the inner side of the wrist : it describes a deep concave curve 

 towards the palmar aspect, and the trapezial portion * is relatively longer than 

 * M. de BlainvUle (Osteographie des Paresseux, 4to, p. 22) prefers to regard this process as the 

 sesamoid of the thumb, notwithstanding that it supports the base of the metacarpal bone ; a deter- 

 mination which is the more singular, because the sesamoid bones are developed to augment the force 

 of the flexor tendons, and here the phalanges which such tendons should bend are wanting, and the 

 rudiment of the thumb in the Ai is itself inflexible. 



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