130 MAMMALS

Porpoises feed on fish, and follow the shoals for days,
being often captured in the fishermen’s nets. Their
peculiarly graceful curves as they rise from the water
to breathe and dive again, and the speed at which
they swim, are well known. The back-fin, so con-
spicuous in the common porpoise, is absent in the
Indian one.

As porpoises feed on fish, so they, in their turn,
are fed on, one of their greatest enemies being one
of their own family, the grampus, which is the only
cetacean preying on cetaceans. A grampus has been
known to swallow four porpoises one after another.
The grampus, Orca gladiator, otherwise the killer
whale, is the powerful swimmer with the big
back-fin, met with in all the world’s seas, and occa-
sionally up many of its rivers. There were even
three killers in the Thames one spring morning
in 1890, disporting themselves off Battersea Park.
They swim in packs, and so fierce are they that they
master the Greenland whale, and will swim into his
mouth to eat out his tongue. The killer has from
forty to fifty teeth, which are much larger than those
of any other dolphin, but are of the usual peg-like
shape, which it has been endeavoured to derive from
the splitting up into threes of some such trilobed
teeth as appear-in a few of the seals.

UNGULATA.—In this order are grouped the
animals whose toe-nails are more or less expanded
into hoofs. It contains a very large number of
species of the first importance in the food-supply of
man. The cattle and sheep, the deer and horses