the banks of the Parana, at the height of a hundred feet, great beds of
an estuary shell, now living a hundred miles lower down nearer the sea;
and I found similar shells at a less height on the banks of the
Uruguay; this shows that just before the Pampas was slowly elevated
into dry land, the water covering it was brackish. Below Buenos Ayres
there are upraised beds of sea-shells of existing species, which also
proves that the period of elevation of the Pampas was within the recent
period.
In the Pampaean deposit at the Bajada I found the osseous armour of a
gigantic armadillo-like animal, the inside of which, when the earth was
removed, was like a great cauldron; I found also teeth of the Toxodon
and Mastodon, and one tooth of a Horse, in the same stained and decayed
state. This latter tooth greatly interested me, [3] and I took
scrupulous care in ascertaining that it had been embedded
contemporaneously with the other remains; for I was not then aware that
amongst the fossils from Bahia Blanca there was a horse's tooth hidden
in the matrix: nor was it then known with certainty that the remains of
horses are common in North America. Mr. Lyell has lately brought from
the United States a tooth of a horse; and it is an interesting fact,
that Professor Owen could find in no species, either fossil or recent,
a slight but peculiar curvature characterizing it, until he thought of
comparing it with my specimen found here: he has named this American
horse Equus curvidens. Certainly it is a marvellous fact in the
history of the Mammalia, that in South America a native horse should
have lived and disappeared, to be succeeded in after-ages by the
countless herds descended from the few introduced with the Spanish
colonists!
The existence in South America of a fossil horse, of the mastodon,
possibly of an elephant, [4] and of a hollow-horned ruminant,
discovered by MM. Lund and Clausen in the caves of Brazil, are highly
interesting facts with respect to the geographical distribution of
animals. At the present time, if we divide America, not by the Isthmus
of Panama, but by the southern part of Mexico [5] in lat. 20 degs.,
where the great table-land presents an obstacle to the migration of
species, by affecting the climate, and by forming, with the exception
of some valleys and of a fringe of low land on the coast, a broad
barrier; we shall then have the two zoological provinces of North and
South America strongly contrasted with each other. Some few species
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