a shell being an obvious
advantage--displaced the curved shells. In the Permian, we saw, a new
and more advanced type of the coiled-shell animal, the Ammonite, made
its appearance, and in the Triassic and Jurassic it becomes the ogre or
tyrant of the invertebrate world. Sometimes an inch or less in diameter,
it often attained a width of three feet or more across the shell, at the
aperture of which would be a monstrous and voracious mouth.

The Ammonites are not merely interesting as extinct monsters of the
earth's Middle Ages, and stimulating terrors of the deep to the animals
on which they fed. They have an especial interest for the evolutionist.
The successive chambers which the animal adds, as it grows, to the
habitation of its youth, leave the earlier chambers intact. By removing
them in succession in the adult form we find an illustration of the
evolution of the elaborate shell of the Jurassic Ammonite. It is an
admirable testimony to the validity of the embryonic law we have often
quoted--that the young animal is apt to reproduce the past stages of
its ancestry--that the order of the building of the shell in the late
Ammonite corresponds to the order we trace in its development in
the geological chronicle. About a thousand species of Ammonites were
developed in the Mesozoic, and none survived the Mesozoic. Like the
Trilobites of the Primary Era, like the contemporary great reptiles
on land, the Ammonites were an abortive growth, enjoying their hour of
supremacy until sterner conditions bade them depart. The pretty
nautilus is the only survivor to-day of the vast Mesozoic population of
coiled-shell Cephalopods.

A rival to the Ammonite appeared in the Triassic seas, a formidable
forerunner of the cuttle-fish type of Cephalopod. The animal now boldly
discards the protecting and confining shell, or spreads over the outside
of it, and becomes a "shell-fish" with the shell inside. The octopus of
our own time has advanced still further, and become the most powerful of
the invertebrates. The Belemnite, as the Mesozoic cuttle-fish is called,
attained so large a size that the internal bone, or pen (the part
generally preserved), is sometimes two feet in length. The ink-bags of
the Belemnite also are sometimes preserved, and we see how it could balk
a pursuer by darkening the waters. It was a compensating advantage for
the loss of the shell.

In all the other classes of aquatic animals we find corresponding
advances. In