60 



The Bradypus tridactylus has but two flexible phalanges in each of the three ungui- 

 culate toes, but this reduction is due to the early anchylosis of the proximal phalanx 

 with the metacarpal, not, as in Megatherium, with the second phalanx. In the Bra- 

 dypus didactylus the unguiculate digits preserve the normal number of free pha- 

 langes. 



The pollex is atrophied in the Megatherium, as it is in both existing species of Sloth ; 

 and, as in the Bradypus tridactylus, only the second, third and fourth digits support 

 claws ; but the fifth digit, instead of being wanting, as in the Ai, is developed, so far as 

 was needed for the purposes of terrestrial progression, in the Megatherium. The small 

 existing arboreal Sloths are seldom obliged to walk on the ground, and there can only 

 crawl along with difficulty. In the Mylodon the pollex was developed and unguiculate, 

 but both the fourth and fifth digits were terminated by a stunted second phalanx. In the 

 Unau, not only the fourth and fifth digits, but also the first are suppressed in the fore- 

 foot. Yet this is the Sloth, as already remarked, which so peculiarly illustrates the bra- 

 dypodal affinities of the Megatherium in the structure of the carpus, notwithstanding 

 the degree to which the adaptive modifications of the Megatherioid type of fore-foot 

 are carried in relation to the exclusively arboreal life of this small existing tardigrade. 

 The coalescence of the scaphoid and trapezium, which Cuvier was the first to recog- 

 nize in the existing Sloths, he continued to affirm in the latest edition of the ' Ossemens 

 Fossiles' to be peculiar to them. The bony structure of the fore-foot of the Megathe- 

 rium he regarded as most resembling that of the Dasypus gigas. M. Laurillard, after 

 the subsequent reception of casts of the carpal bones of the Megatherium, which had 

 been transmitted to England, with other bones of the Megatherium, by Sir Woodbine 

 Parish, K.H., inferred that the fore-foot of the Megatherium had a greater analogy with 

 that of the Myrmecophaga jubata, but he did not detect the connation of the scaphoid 

 with the trapezium. The form of the scapho-trapezial bone in both existing Sloths 

 bears an unmistakeable resemblance to that in the Megatherium, but in the Unau it 

 describes a deeper curve towards the palmar aspect, and the trapezial portion (described 

 by DeBlainville as the sesamoid of the pollex*) is relatively longer than in the Mega- 

 therium. The base of the stunted metacarpal of the pollex is expanded, and abuts by 

 one part against the trapezium and by another against the base of the second meta- 

 carpal. The trapezoides is a small bone articulated, as in the Megatherium, with the 

 scapho-trapezial, the os magnum, and the second metacarpal ; the os magnum presents 

 almost the same pentagonal contour, dorsally, as in the Megatherium, the anterior facet 

 being also partly convex for adaptation to a concavity in the base of the middle meta- 

 carpal, which likewise is so extended as to interpose itself between the fourth meta- 

 carpal and the carpus. 



The atrophy of the fifth finger, which has proceeded in the Megatherium to cause 

 the absence of the ungual phalanx, and which atrophy similarly affects both the 

 fifth and fourth digits in the Mylodon and Scelidotherium, has proceeded in the 



• Ost6ographio de Pareaseux, 4to, p. 22. 



