or stone lilies, some of which consisted of but a sculptured
calyx without petals, while others threw off a series of long, flexible
arms, that divided and subdivided like the branches of a tree, and were
thickly fringed by hair-like fibres. There is great variety and beauty
among these Silurian crinoids; and, from the ornate sculpture of their
groined and ribbed _capitals_ and slender _columns_, the Gothic
architect might borrow not a few striking ideas.

[Illustration: Fig. 46.

CYATHAXONIA DALMANI.]

The difference between the older and newer fashions, as exemplified in
the cup-shaped corals, may be indicated in a single sentence. The
ancient corals were stars of four rays, or of multiples of four; the
modern corals are stars of six rays, or of multiples of six. But though,
at a certain definite period,--that during which the great Palaeozoic
division ended and the Secondary division began--nature, in forming
this class of creatures, discarded the number four, and adopted instead
the number six, the great leading idea of the star itself was equally
retained in corals of the modern as in those of the more ancient type.

[Illustration: Fig. 47.

GLYPTOCRINUS DECADACTYLUS.

(Hudson River Group, Lower Silurian.)]

[Illustration: Fig. 48.

CALYMENE BLUMENBACHII.]

[Illustration: Fig. 49.

ORTHISINA VERNEUILI.]

[Illustration: Fig. 50.

LITUITES CORNU-ARIETIS.]

[Illustration: Fig. 51.

LINGULA LOWISII.]

The articulata of the Silurian period bore a still more peculiar
character. They consisted mainly of the Trilobites,--a family in whose
nicely-jointed shells the armorer of the middle ages might have found
almost all the contrivances of his craft, anticipated, with not a few
besides which he had failed to discover; and which, after receiving so
immense a development during the middle and later times of the Silurian
period, that whole rocks were formed almost exclusively of their
remains, gradually died out in the times of the Old Red Sandstone, and
disappeared for ever from creation after the Carboniferous Limestone had
been deposited. The Palaeontologist knows no more unique family than that
of the Trilobites, or a family more unlike any which now exists, or a
family which marks with more certainty the early rocks in which they
occur. And yet, though formed in a fashion that perished myriads of
ages ago, how admirably does it not exhibit the articulated type of
being, and illustrate that unity of de