the remaining Molluscs the higher or more effective types
are displacing the older. It is interesting to note that the oyster is
fully developed, and has a very large kindred, in the Mesozoic seas.
Among the Brachiopods the higher sloping-shoulder type displaces the
square-shoulder shells. In the Crustacea the Trilobites and Eurypterids
have entirely disappeared; prawns and lobsters abound, and the earliest
crab makes its appearance in the English Jurassic rocks. This sudden
arrival of a short-tailed Crustacean surprises us less when we learn
that the crab has a long tail in its embryonic form, but the actual
line of its descent is not clear. Among the Echinoderms we find that the
Cystids and Blastoids have gone, and the sea-lilies reach their climax
in beauty and organisation, to dwindle and almost disappear in the last
part of the Mesozoic. One Jurassic sea-lily was found to have 600,000
distinct ossicles in its petrified frame. The free-moving Echinoderms
are now in the ascendant, the sea-urchins being especially abundant.
The Corals are, as we saw, extremely abundant, and a higher type (the
Hexacoralla) is superseding the earlier and lower (Tetracoralla).

Finally, we find a continuous and conspicuous advance among the fishes.
At the close of the Triassic and during the Jurassic they seem to
undergo profound and comparatively rapid changes. The reason will,
perhaps, be apparent in the next chapter, when we describe the gigantic
reptiles which feed on them in the lakes and shore-waters. A greater
terror than the shark had appeared in their environment. The Ganoids and
Dipneusts dwindle, and give birth to their few modern representatives.
The sharks with crushing teeth diminish in number, and the sharp-toothed
modern shark attains the supremacy in its class, and evolves into forms
far more terrible than any that we know to-day. Skates and rays of a
more or less modern type, and ancestral gar-pikes and sturgeons,
enter the arena. But the most interesting new departure is the first
appearance, in the Jurassic, of bony-framed fishes (Teleosts). Their
superiority in organisation soon makes itself felt, and they enter upon
the rapid evolution which will, by the next period, give them the first
place in the fish world.

Over the whole Mesozoic world, therefore, we find advance and the
promise of greater advance. The Permian stress has selected the fittest
types to survive from the older order; the Jurassic luxuriance is
pe