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.
The “column” is the stem by which the
animal is attached permanently to the bottom of the
sea; and it is composed of numerous separate plates,
so jointed together that whilst the amount of movement
between any two pieces must be very limited, the entire
column acquires more or less flexibility, allowing
the organism as a whole to wave backwards and forwards
on its stalk. Into the exquisite
minutioe
of structure by which the innumerable parts entering
into the composition of a single Crinoid are adapted
for their proper purposes in the economy of the animal,
it is impossible to enter here. No period, as
before said, has yielded examples of greater beauty
than the Upper Silurian, the principal genera represented
being
Cyathocrinus, Platycrinus, Marsupiocrinus,
Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus,
Periechocrinus, Glyptocrinus, Crotalocrinus, and
Edriocrinus.
[Illustration: Fig 62.—Upper Silurian
Crinoids. a, Calyx and arms of Eucalyptocrinus
polydactylus, Wenlock Limestone; b, Ichthyocrinus
loevis, Niagara Limestone, America; c, Taxocrinus
tuberculatus, Wenlock Limestone. (After M’Coy
and Hall.)]
[Illustration: Fig. 63.—Planolites
vulgaris, the filled-up burrows of a marine worm.
Upper Silurian (Clinton Group), Canada. (Original.)]
The tracks and burrows of Annelides are as
abundant in the Upper Silurian strata as in older
deposits, and have just as commonly been regarded
as plants. The most abundant forms are the cylindrical,
twisted bodies (Planolites), which are so frequently
found on the surfaces of sandy beds, and which have
been described as the stems of sea-weeds. These
fossils (fig. 63), however, can be nothing more, in
most cases, than the filled-up burrows of marine worms
resembling the living Lob-worms. There are also
various remains which belong to the group of the tube-inhabiting
Annelides (Tubicola). Of this nature are
the tubes of Serpulites and Cornultites,
and the little spiral discs of Spirorbis Lewisii.