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Unau to the removal of all the. bones of those digits, save the metacarpal of the 

 fourth, which is reduced to a rudiment of even smaller size than that which forms 

 the vestige of the thumb on the radial side of the hand : it rests, as a great part of the 

 fourth metacarpal in the Megatherium does, upon the expanded base of the third meta- 

 carpal. 



In the Ai (Bradypus tridactylus) the metacarpal rudiment of the pollex is anchylosed 

 at its lateral joint to the base of the metacarpal of the index, but it retains its free arti- 

 culation with the scapho-trapezium. The chief modifications of both hand and foot in 

 the Three-toed Sloth are the extensive anchyloses of different bones : this character is 

 shown by the coalescence of the trapezoides with the os magnum, such compound bone 

 supporting the base of the second metacarpal and a great part of that of the middle 

 metacarpal; thus fulfilling the same relations to the metacarpus as do the separated 

 bones in the Unau and Megatherium. 



The great extent to which the metacarpals are suturally united to each other in the 

 Megatherium, is a character repeated in those of the Ai, but the suture is speedily, in 

 the living Sloth, converted into bony union, and the three metacarpals, like the three 

 metatarsals, thus form one compound bone, as in Birds. The unguiculate digits which 

 this bone supports in the fore-paw, are the homologues of the three claw-bearing toes in 

 the Megatherium. The rudiment of the fifth finger appears as a mere process from the 

 outside of the base of the metacarpal of the fourth : the huge terrestrial predecessor of 

 the small leaf-eating and tree-dwelling quadrupeds retained the fifth toe, minus its ter- 

 minal phalanx, yet of great size and strength, and modified expressly for the purpose of 

 supporting the ponderous body in terrestrial progression. 



The fore-foot of the Mylodon more closely conforms, in its essentials, to the type of 

 that of the Unau, inasmuch as the two outer digits (fourth and fifth) were mutilated 

 and clawless ; they were, however, developed to the same degree as the fifth digit is in 

 the Megatherium, and for the same end, but probably made little show, externally, in 

 the entire foot. The pollex, however, instead of being rudimental, was fully developed, 

 though small, in the Mylodon. In the Megatherium this digit is rudimental, as in both 

 forms of existing Sloth ; but the bones of the fore-foot correspond more closely with 

 the type of the manus in the Ai ; there being, indeed, as, regards the digits, only this 

 essential difference, that the fifth, instead of being, like the first, a mere rudiment, was 

 developed to be adapted to progression on the ground. It is most interesting, however, 

 to trace the interchangeable relations between the two above-cited great extinct Mega- 

 therioids and the two existing forms of Sloth, respectively. 



In regard to other existing Edentata, the Myrmecophaga jubata, by reason of the 

 clawless condition of its fifth digit, and the Myrmecophaga didactyla, by that of the 

 rudimental pollex as well as fourth and fifth digits, ought to succeed the Sloths as next 

 of kin to the Megatherioid quadrupeds, the interval being due to the difference of carpal 

 structure. 



Cuvier has observed that the fore-foot of the Dasypus gigas is one of the most extra- 



