192 THE ANIMALS OF LAKES AND RIVERS 
not very profound, not sufficient to cause the animals 
to diverge very markedly from their terrestrial allies. 
This suggests that in most cases the modification is 
relatively recent. Among the more frequent modifica- 
tions are webbed feet, used in swimming, close dense 
fur which prevents the water reaching the skin and so 
chilling it, and a broad flattened tail used as a rudder. 
Of mammals with more profound adaptations to 
aquatic life, few occur in inland waters. We have 
already spoken of the Lake Baikal seal (Phoca sibirica) 
and the Caspian seal (Phoca caspica), both closely 
related to the Ringed seal of northern waters. The 
Baikal seal also occurs in Lake Oron, on the course of 
the river Vitim, and in addition to its seal Lake Baikal 
shows certain ‘marine’ features in its molluscan fauna. 
The explanation is perhaps that the lake had once 
a connexion, direct or indirect, with the great inland 
sea of Tertiary times. Among the Sirenia the manatee 
is largely fresh-water, ascending such rivers as the 
Amazon almost to their sources. But these animals 
thrive equally well in the ocean, and their presence in 
large rivers requires no special explanation. 
The large Cetaceans are exclusively marine, but 
certain dolphins occur in fresh water. The one which 
is most definitely adapted for this habitat is the blind 
dolphin of the Ganges, Brahmaputra, and Indus 
(Platanista gangetica). This animal occurs from tidal 
waters to the upper reaches of the rivers, as high up 
as the depth of the water permits, but it has never 
been seen in the sea. These dolphins feed upon fresh- 
water fish and river prawns, and are apparently primi- 
tive forms which, like Ganoid fish, have found a refuge 
in fresh water. Other fresh-water dolphins occur in the 
large rivers of South America, especially the Amazon. 
