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.
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