ubes
of _Halysites agglomerata_, of the natural size; d, Vertical
section of two tubes of the same, showing the tabulae, enlarged.
Niagara Limestone (Wenlock), Canada. (Original.)]

[Illustration: Fig. 60.--Upper Silurian Star-fishes. 1, _Palasterina
primoeva_, Lower Ludlow; 2, _Paloeaster Ruthveni_, Lower Ludlow;
3, _Paloeocoma Colvini_, Lower Ludlow. (After Salter.)]

[Illustration: Fig. 61.--A, _Protaster Sedgwickii_, showing the
disc and bases of the arms; B, Portion of an arm, greatly enlarged.
Lower Ludlow. (After Salter.)]

Amongst the _Echinodermata_, all those orders which have hard parts
capable of ready preservation are more or less largely represented.
We have no trace of the Holothurians or Sea-cucumbers; but this
is not surprising, as the record of the past is throughout almost
silent as to the former existence of these soft-bodied creatures,
the scattered plates and spicules in their skin offering a very
uncertain chance of preservation in the fossil condition. The
Sea-urchins (_Echinoids_) are said to be represented by examples
of the old genus _Paloechinus_. The Star-fishes (_Asteroids_) and
the Brittle-stars (_Ophiuroids_) are, comparatively speaking,
largely represented; the former by species of _Palasterina_ (fig.
60), _Paloeaster_ (fig. 60), _Paloeocoma_ (fig. 60), _Petraster,
Glyptaster_, and _Lepidaster_--and the latter by species of
_Protaster_ (fig. 61), _Paloeodiscus, Acroura_, and _Eucladia_.
The singular _Cystideans_, or "Globe Crinoids," with their globular
or ovate, tesselated bodies (fig. 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 sm