NOTICE OF THE MEGATHERIUM. 
17 
with the Great Anteater. The Three-toed Sloth has two more cervical vertebrae, while the 
Two-toed has eight additional dorsals, the greatest number known in any Mammalian quad, 
ruped. All the Edentates have ossified sternal ribs like the Megatherium, but the Sloths 
come nearest in having their first three ribs anchylosed to their sternal complements. Only 
the Sloths and Anteaters resemble the Megatherium in the small number of the lumbar 
vertebrae ; the latter have also the same number in the sacral region. The part of the 
skeleton in which the Megatherium differs most from the Sloths is the tail, which in these 
latter is so short as to be hardly visible. In the terrestrial Edentates, on the other hand, 
there is the same extended and complex development of that member which we have 
observed in the ground-going fossil. 
The scapula of the Megatherium, in the confluence of the acromium with the coracoid, 
exclusively resembles that of the Sloth. In its clavicle and carpus it approaches nearest to the 
Unau ; in the bones of the fore-foot it corresponds more closely with the Ai. The characters 
of the humerus and ulna are most closely repeated in the Great An- eater. The fore-legs of 
the Sloth are nearly twice the length of the hind ones, and in this respect differ from the Meg- 
atherium. All the Edentates have the ischiatic notch nearly closed with solid bone, like the 
Megatherium, but the Sloths alone imitate the expanded ilia. They also repeat the wide flat- 
tening of the femur with the absence of its medullary cavity* The Sloths have the prolon- 
gation of the heel, and, alone among living Edentates, suppress certain toes. The Megathe- 
rium presents alike an exception to the rule that unguiculated mammals have penta-dactyle 
feet ; since not even a vestige of the inner toe exists. The conclusion of this zoological com- 
parison is, that the Megatherium finds its nearest living analogue in the Sloth family, of the 
order of Edentates. It agrees with the Sloth chiefly in the structure and growth of its teeth ; 
it differs chiefly in the peculiar length of the pre-maxillaries, and in the possession of a long 
tail. Being neither a climber nor a burrower, its position is expressed by the term applied to 
it by Owen — “The Giant Ground-Sloth of America.” The species under consideration is 
named M. (Jnvieri by Demarest, and M. Americanum by Owen ; while the species found in 
North America is distinguished by Leidy as the M. mirahile. The new primary group, or 
tribe, of leaf-eating edentates, which Owen has established under the name of Phyllophaga, 
includes, together with the living Sloths, the fossil genera, Megalonyx, Megatherium, Mylodon, 
and Scelidotherium. The former and the latter of these fossils are known only by a few frag- 
mentary portions of their skeletons, which are, however, still sufficient to indicate their place 
with the two other ponderous forms, and with them to constitute the natural family to which 
the same author has given the expressive appellation of Grayigrada. 
The Mylodon, — of which a very perfect skeleton was fonnd in the same Pampean formation 
of Brazil, — approached the Megatherium very closely in general form, although it was only 
about two thirds as large. Its skull was very similar, save in the fact that it lacked the 
extended pre maxillary bones, and the lower jaw was not so largely developed downwards for 
* The anchylosed condition of the tibia and fibula, as seen in the Megatherium, is known among existing 
quadrupeds only in armor -bearing Edentates. The hemispherical excavation of the articular surface on the 
distal end of the tibia is peculiar to the Megatherium and the Mylodon, nor has any living Mammal the astrag- 
alus of the Megatheroid quadrupeds, — it being the most characteristic single bone in the skeleton. 
4 
