74 
SYSTEM OF NATURE. 
4thly. Natantia. In the swimming or ichthyoid pla- 
centals the legs are transformed into fins, and have become 
organs of progressive motion in water. These animals we 
find so completely assuming the figure and habits of fishes, 
that there can be no necessity for dwelling on a similarity 
so obvious and so universally admitted. 
5thly. Ambulantia. In this tribe, the feet, instead of 
being divided exteriorly into fingers or toes, as in most of 
the other tribes, are bound together in a solid mass, the 
extremity of which is furnished with one or more horny 
bodies called hoofs. The mode of progression is emi¬ 
nently terrestrial, and by an alternate movement of the 
legs : it may also be said to be eminently digitigrade, the 
toes or hoofs alone coming in contact with the ground. 
The two pairs of feet closely resemble each other, the fore 
pair never being used as organs of prehension. The skin 
is remarkable for its thickness ; whence the name of Pa- 
chydermata. This tribe appears to me to indicate an ap¬ 
proach to the tortoises. The similarity of the solid foot is 
very remarkable, as also the close correspondence of the 
two pairs of feet, and the utter impossibility of the appli¬ 
cation of either pair to purposes of prehension or flight. 
The only modification which the feet ever undergo is the 
transformation into paddles, and consequent adaptation to 
progressive motion in water. 
fitlily. Repentia. In this group the legs are remarkable 
for their shortness, the bellies of the animals nearly touch¬ 
ing the ground: this peculiarity gives them a crawling, 
herpetoid or reptile gait and appearance, greatly enhanced 
by the scaly covering in which many species are encased. 
There appear to be two divisions of this tribe, the ar- 
madilloes and the anteaters; the former “ are completely 
