Small forests of these sea-lilies
adorn the floor of the Silurian ocean, and their broken and dead
frames form whole beds of limestone. The primitive Cystids
dwindle and die out in the presence of such powerful competitors.
Of 250 species only a dozen linger in the Silurian strata, though
a new and more advanced type--the Blastoid--holds the field for a
time. It is the age of the Crinoids or sea-lilies. The starfish,
which has abandoned the stalk, does not seem to prosper as yet,
and the brittle-star appears. Their age will come later. No
sea-urchins or sea-cucumbers (which would hardly be preserved)
are found as yet. It is precisely the order of appearance which
our theory of their evolution demands.
The Brachiopods have passed into entirely new and more advanced
species in the many advances and retreats of the shores, but the
Molluscs show more interesting progress. The commanding group
from the start is that of the Molluscs which have "kept their
head," the Cephalopods, and their large shells show a most
instructive evolution. The first great representative of the
tribe is a straight-shelled Cephalopod, which becomes "the tyrant
and scavenger of the Silurian ocean" (Chamberlin).
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