126 ' ' MAMMALS

rest with his mouth wide open, and snaps his jaws
together when anything he fancies comes swimming
into them. His top speed is twelve knots, and when
he rises to blow he is about twelve minutes at the
surface, and he dives to enormous depths, having
been known to run out nearly a mile of line when
harpooned. This is the large sperm- -whale or cachalot,
Physeter macrocephalus ; the small sperm-whale, Cogda
breviceps, is more like a porpoise. The other sperm-
whales are found only as fossils, all from comparatively
recent rocks, the whales apparently not dating vely,
far back in the past. F

The xiphioid whales have only two or four teeth
in the lower jaw, the most familiar of this group being
the North Atlantic bottlehead (Hyperiodon rostratus),
which has but two, and these hidden inthe gum. The
bottlehead, so called from the lump in front of the
blow-hole, is about thirty feet long ; he is black when
he is young, and turns yellow and white as he
grows old; and, like the sperm-whale, he yields sperm-
oil and spermaceti, or substances so similar to them
that they answer the same purposes. He does not
always keep to the open sea: he is frequently found
in the English Channel, and he was once caught above
London Bridge. Like all the whales, he has not much
of a neck to speak of, but like all of them, and like
nearly all other mammals, he has just as many
vertebree in it—seven—as there are in the neck of the
giraffe.

The dolphins and porpoises are all grouped as
Delphinide. There are a large number of them,
most of them being marine, some estuarine, and a few