242 Royal Society. 



{Mi/rmecophagce): but it does not resemble the Armadillos (/>a5^/?M5) 

 ia having long metapophyses, the peculiar development of which in 

 those loricated Bruta has a direct relation to the support of their 

 bony dermal armour. In tlie mesozygapophyses of the middle 

 dorsal vertebrae the Megatherium is peculiar. In the small extent 

 of the produced and pointed symphysis pubis it resembles the Sloths; 

 and in the junction of both ilium and ischium with the sacrum, it 

 manifests a character common to the Edentate order ; but in the 

 expanse and massiveness of the iliac bones, it can only be compared 

 with other extinct members of its own peculiar family of Phyllo- 

 phagous Edentata. Its habits necessitating a strong and powerful 

 tail, we find this resembling in its bony structure that of other 

 Edentata with a similar appendage, especially in the independency 

 of the two hoemapophyses of the first caudal, a character which ob- 

 tains in the Great Ant-eater and in some Armadillos; but this is 

 no evidence of direct affinity to either of these families ; the habits 

 of the small arboreal Sloths render their eminently prehensile limbs 

 sufficient for their required movements, and the tail is wanting. 

 Had that appendage been proportionally as large as in the Mega- 

 therium, we cannot suppose that the caudal vertebrae would have 

 materially differed from those of other Edentata. 



In the coalescence of the anterior vertebral ribs with the bony 

 sternal ribs, the Megatherium resembles the Sloths. This essential 

 affinity is still more marked in the peculiarities of the scapula and 

 of the carpus. In the Mi/rmecophagajubata, the scaphoid is distinct: 

 in the Manis it coalesces with the lunare : in the Dasypus gigas 

 the trapezoides is anchylosed to the second metacarpal : in the Das, 

 sexcinctus it has coalesced with the trapezium. Not any of these 

 characteristics are manifested by the Megatherium : its carpus re- 

 peats the peculiarities of that in the Sloths, viz. the reduction of the 

 number of carpal bones to seven by the coalescence of the scaphoid 

 with the trapezium. The first digit (poUex), which is retained in 

 the Anteatcrs and Armadillos, is obsolete in the Megatherium as 

 in the Sloths and Orycteropus : three digits are fully developed and 

 armed with claws, as in the JBradi/pus tridactylus; and the fifth, 

 though incomplete in the Megatherium, is better developed, because 

 it was required in the ponderous terrestrial Sloth for its progression 

 on level ground. In no existing ground-dwelling Edentate is the 

 fifth digit deprived of its ungual phalanx, as in the Megatherium. 

 The bones of the fore-foot of that extinct animal are thus seen to be 

 modified mainly after the type of the BradypodidcB. 



The long bones of all the limbs are devoid of medullary cavities, as 

 in the Sloths. The femur lacks the ligamentum teres as in the Sloths. 

 The fibula is anchylosed to the tibia at both ends in Megatherium, 

 as in Dasypus \ but this is not the case in the closely- allied extinct 

 Megatherioids called Mylodon^ Megcdonyx and Scelidotherium, a 

 fact which diminishes the force of the argument which Cuvier de- 

 duced from the coalesced condition of the bones in the Megatherium 

 in favour of its affinities to the Armadillos. The semi-inverted but 

 firm interlocking articulation of the hind-foot to the leg shows the 

 peculiarities of that joint in the Sloths exaggerated, and departs 



