822 
PEOFESSOE OWEIS ON THE MEGATHEETOI. 
that for the cuboides is continuous ; the posterior part of the astragalar surface is less 
convex than in the Megatherium : the posterior prolongation of the calcaneum is rela- 
tively shorter and is less pointed : the posterior wall of the great outer tendinal groove 
is more developed in the Mylodon. In the Scelidotheiium the posterior termination of 
the calcaneum is broader, and terminated by a less angular convexity than in the Mylo- 
don ; but in the separation of the posterior from the anterior part of the astragalar 
surface it agrees with the Megatherium and with the Sloths. These, of ah recent 
Bruta, most resemble the Megatherioids in the posterior prolongation of the calca- 
neum, and more especially the Megalonyx in the compression and distal expansion of 
that lever ; but, in the Megalonyx, this expansion in the vertical direction is extreme, 
and gives the heel-bone more the form of an os ilium than of a tarsal bone. 
In the os cuboides of the Mylodon the two surfaces for the fourth and fifth metatarsals 
are nearly on the same plane ; in the Megatherium they are nearly at a right angle. 
In the Scelidotheiium the surface by which the cuboides joins the astragalus is convex, 
instead of concave, as in the Megatherium and Mylodon. In the latter genus the sur- 
face on the os naviculare for the mesocuneiform bone is relatively broader than in the 
Megatherium ; and the upper division of the astragalar surface is flat, instead of being 
convex. 
With respect to the digits, it is significant of the true afiinities of the Megatherium 
to find that the Sloths alone amongst the existing JBruta show a suppression of certain 
toes: in all the other existing genera of the order the hind foot retains the typical 
number of digits. 
The Armadillos, including the CMamyphorus, resemble the Megatherium in the ter- 
minal confluence of the tibia and fibula ; but these bones articulate in the Basyjjodich^ 
in the normal way with the astragalus, and the foot is planted firmly on the ground by 
its flat surface. The hallux is not only present in the Chlciiny^pliofus, but is longer than 
the fifth toe. All the five toes are armed with claws ; and anomalous as the three-toed 
hind feet of the Sloths may appear, they have more real resemblance vrith the feet of 
the Megatherioids than have those of any of the Armadillo family. The great extinct 
Glyptodon agrees with the Chlamyphorus in the pentadactyle condition of the hind foot, 
the os naviculare having three facets for as many cuneiform bones, the innermost of 
which supports a well-developed hallux. This toe is abortive in existing Sloths, and is 
represented by a small compressed bone, consisting apparently of the connate ento- 
cuneiform and metatarsal : it retains its distinctness in the Unau, as the similar repre- 
sentative of the second toe does in the Megatherium ; and the meso- and ecto-cuneiform 
bones are also distinct ; but in the Ai each cuneiform bone is confluent uith its meta- 
tarsal, with each other, with the naviculare, and with the cuboides. 
In the Megatherium, the mutilation of the foot has commenced on its outer side by 
the removal of the ungual phalanx from the fifth arrd fornth toes ; in the Sloths the 
mutilation is restricted to the fifth toe, but is carried fiu’ther, viz. to the removal of all 
the phalanges and the reduction of size of the metatarsal: nevertheless, the Sloths alone, 
