of the Sloths to their peculiar Mode of Life. 21 
that it never can be placed flat or have a firm tread upon the ground, but if set 
on the earth would rest on its outer edge. Now there is not one of these 
peculiarities that is not admirably adapted to render complete the mechanical 
power of the hind-leg and foot as organs by which the animal is enabled to 
attach itself most firmly, and as it were with pincers and grappling-hooks, to 
the trunk and branches of a tree. : 
The unusual stiffness of the toes and fingers is another peculiarity of the Ai, 
not less fitted to assist its habits of constantly living and feeding upon trees ; 
all the bones of the fingers and toes, except the claw-bones, are inclosed in an 
undivided skin, and can only move together; the claws alone are separate. 
The first joints of the fingers and toes are united to those of the metatarsus and 
metacarpus; the bones of the metacarpus also being consolidated with them 
into a single piece, which represents what in many animals are 14 small bones. 
In the hind-foot there is a similar union of the first joints with the bones of 
the metatarsus, one bone representing what in the more active animals are 17. 
This stiffness of the parts, which would be inconvenient to an animal moving 
on the ground, becomes advantageous and a source of strength to one whose 
constant position and occupation are almost stationary upon a tree. The 
claws of the Sloth are of unusual length, and form a powerful instrument of 
defence; with these a Sloth has been known to strangle a dog, holding him 
at arm’s length: on trees also it is most surprisingly tenacious of its hold, and 
the limbs, though possessing great capability of motion, can fix themselves 
almost with the rigidity of iron. Mr. Burchell has seen the limbs, even just 
after death, continue fast clinging round the object to which they were 
adhering before the animal expired. 
The Sloth has till very recently been supposed to present a most extraordi- 
nary deviation from the normal character of all Mammalia in the number of 
its cervical vertebrze; all other Mammalia, from the Giraffe and Camel, down 
to the Cetacea, have invariably seven, while the Sloth was considered.to have 
nine. Mr. T. Bell* has lately ascertained, by the dissection of two specimens 
of the Bradypus tridactylus, that the two lowest of these supposed cervical ver- 
tebræ are in reality dorsal, having two small and short rudimentary ribs 
attached to each of them, which bave been hitherto overlooked in the dissec- 
* -See paper read before the Zoological Society of London, August 13th, 1833. 
