824 
PEOrESSOE OWEN ON TBCB jVIEGATHEEnJW. 
in those loricated Bruta has a direct relation to the support of their bony dermal armour. 
In the mesozygapophyses of the middle dorsal vertebrae*, the Megatherium is pecu- 
liar amongst Mammalia. In the small extent of the produced and pointed symphysis 
pubis it resembles the Sloths ; and in the junction of both ihum 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 phyllophagous Bruta. The habits of the Megathere 
necessitating a strong and powerful tail, we find this resembling in its bony structure 
that of other Bruta with a similar appendage, especially in the independency of the two 
hgemapophyses of the first caudal, a character which obtains in the Amteatersf and in 
some Armadillos ; but this is no evidence of dhect affinity to either of those famihes : 
the habits of the small arboreal Sloths render their eminently prehensile limbs sufi5.cient 
for their required movements, and the tail is wanting. Had that appendage been pro- 
portionally as large as in the Megatherium, we cannot suppose that the caudal vertebrae 
would have materially differed from those of other Bruta. 
In the coalescence of the anterior vertebral ribs with the bony sternal ribs, the Mega- 
therium resembles the Sloths. This essential affinity is still more marked in the pecu- 
liarities of the scapula and of the carpus. In the Myrmecophaga jubata, the scaphoid is 
distinct: in the Manis it coalesces with the lunare: in the Basypus 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 repeats 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first digit (pollex), which is retained in the Anteaters and Armadillos, is obsolete in the 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 Bradypus tridactylus ; and the fifth, though incomplete in 
the Megatherium, is better developed, because it was requhed in the ponderous terres- 
trial Sloth for its progression on level ground. In no existing ground-dwelling member 
of the Bruta-i^ the fifth digit deprived of its ungual phalanx, as m the Megatherium. 
The bones of the fore foot of that extinct animal are thus seen to be modified mainly 
after the type of the Bradypodidce. 
The long bones of all the limbs are devoid of medullary carities, 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 Basypus; but this is not the case in the closely- 
allied extinct Megatherioids called Mylodoii, Megalonyx, and Scelidotheiium, a fact 
which diminishes the force of the argument which Cuvier deduced 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 further hour its 
characteristics in other Bruta. In all the existing members of the order, save the 
* Philosophical Transactions, 1855, Plate XIX. mz. t Ihid. 1851, Plate LIII. fig. 60. 
