46, A, C, D,), are also not
uncommon in the Upper Silurian; and if they do not become finally
extinct here, they certainly survive the close of this period by
but a very brief time. By far the most important, however, of
the Upper Silurian Echinodenns, are the Sea-lilies or _Crinoids_.
The limestones of this period are often largely composed of the
fragmentary columns and detached plates of these creatures, and
some of them (such as the Wenlock Limestone of Dudley) have yielded
perhaps the most exquisitely-preserved examples of this group
with which we are as yet acquainted. However varied in their
forms, these beautiful organisms consist of a globular, ovate,
or pear-shaped body (the "calyx"), supported upon a longer or
shorter jointed stem (or "column"). The body is covered externally
with an armour of closely-fitting calcareous plates (fig. 62),
and its upper surface is protected by similar but smaller plates
more loosely connected by a leathery integument. From the upper
surface of the body, round its margin, springs a series of longer
or shorter flexible processes, composed of innumerable calcareous
joints or pieces, movably united with one another. The arms are
typically five in number; but they generally subdivide at least
once, sometimes twice, and they are furnished with similar but
more slender lateral branches or "pinnules," thus giving rise
to a crown of delicate feathery plumes.
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