Favistella, Streptelasma_, and _Zaphrentis_; whilst amongst the
"Tabulate" Corals, the principal forms belong to the genera
_Choetetes, Halysites_ (the Chain-coral), _Constellaria_, and
_Heliolites_. These groups of the Corals, however, attain a greater
development at a later period, and they will be noticed more
particularly hereafter.

[Illustration: Fig. 46.--Group of Cystideans. A, _Caryocrinus
ornatus_,[13] Upper Silurian, America; B, _Pleurocystites squamosus_,
showing two short "arms," Lower Silurian, Canada; C, _Pseudocrinus
bifasciatus_, Upper Silurian, England; D, _Lepadocrinus Gebhartii_,
Upper Silurian, America. (After Hall, Billings, and Salter.)]

[Footnote 13: The genus _Caryocrinus_ is sometimes regarded as
properly belonging to the _Crinoids_, but there seem to be good
reasons for rather considering it as an abnormal form of
_Cystidean_.]

Passing onto higher animals, we find that the class of the
_Echinodermata_ is represented by examples of the Star-fishes
(_Asteroidea_), the Sea-lilies (_Crinoidea_), and the peculiar
extinct group of the Cystideans (_Cystoidea_), with one or two of
the Brittle-stars (_Ophiuroidea_)--the Sea-urchins (_Echinoidea_)
being still wanting. The Crinoids, though in some places extremely
numerous, have not the varied development that they possess in
the Upper Silurian, in connection with which their structure will
be more fully spoken of. In the meanwhile, it is sufficient to
note that many of the calcareous deposits of the Lower Silurian
are strictly entitled to the name of "Crinoidal limestones,"
being composed in great part of the detached joints, and plates,
and broken stems, of these beautiful but fragile organisms (see
fig. 12). Allied to the Crinoids are the singular creatures which
are known as _Cystideans_ (fig. 46). These are generally composed
of a globular or ovate body (the "calyx"), supported upon a short
stalk (the "column"), by which the organism was usually attached
to some foreign body. The body was enclosed by closely-fitting
calcareous plates, accurately jointed together; and the stem was
made up of numerous distinct pieces or joints, flexibly united
to each other by membrane. The chief distinction which strikes
one in comparing the Cystideans with the Crinoids is, that the
latter are always furnished, as will be subsequently seen, with
a beautiful crown of branched and feathery appendages, springing
from the summit of the calyx, and which are compos