254 FISHES

thirty feet long; the hammerheads (Zygena), with
the curious right and left extensions of the head,
are smaller but no less formidable. The Carcharodon
may be forty feet long, but from a number of teeth
dredged up by the Challenger, some of which were four
inches across the base, it is supposed that a species
of the genus over seventy feet long must be either
living or only recently extinct. The thresher shark
(Alopecias), with an enormous development of the
main lobe of the tail, is about fifteen feet long, and .
_ is as harmless to man as the basking shark (Selache
maxima), which never attacks man unless in self-
defence. The cestracionts, or Port Jackson sharks,
are of great interest as being the direct descendants
of primary forms, though only three genera out of
some five and twenty have as yet been found living.
With the smaller sharks are grouped the dog-fishes,
that by way of the angel-fish and the Pristiophoride
lead on to the rays, the gap being filled by the saw-
fishes. ;
The saw-fish has the long body of a shark and
the ventral gill openings of a ray. His saw, like the
sword of xiphias, is a long, flattened, bony snout,
but it is double-edged and covered with a rough skin,
and its score or more of serrations on each side may
be looked upon as its false or pioneer teeth, the true
teeth within its mouth being flat and grinding. The
body may be a dozen feet long, and to this is to be
added the saw, which may measure five. This saw
exists in a rudimentary form in all the rays. It is
smooth when the fish is young, the teeth appearing
as the animal advances in age. It is well known as