252 FISHES

hinted, is the oldest living genus; but there were
dipnoids in Devonian days.

ELASMOBRANCHII—These are the cartila-
ginous fishes ; the least specialised and most ancient
of all. There is now in Japanese waters the oldest
of living fishes—a shark named Chlamydoselachus,
with direct ancestors of Devonian age ; but the order
goes even further back, for we find it represented in
Upper Silurian strata) The only bony structures
worth mentioning in the order are the teeth and
scales, the skeleton being almost entirely gristly or
cartilaginous; and here it is we find the placoid
scales, or skin teeth, which are plates tipped with
enamel and based with bone. Some of these fishes
use egg-purses for their ova, while others are vivi-
parous. There are three main divisions of the order
—the chimeras, the sharks, and the rays. The
chimeras have but one external gill opening, and
that is covered by a fold of the skin. They furnish
the intermediate forms between the sharks and the
ganoids, the best known of them being the king ot
the herrings (Chimera monstrosa), a naked fish of
surpassing ugliness. The sharks and rays have trans-
verse mouths and from five to seven gill-openings,
sharks having them on the sides of the body, while
the rays have them underneath—the sharks being of
the usual fish-like form, while the rays are flat, like
the skate. Sharks have no scales, their skin being
covered with the calcified papillee which, when small,
distinguish the ‘shagreen’ of commerce.

The blue sharks (Carcharias) are occasionally