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 composed of innumerable
calcareous plates or joints, and are known as the "arms." In the
Cystideans, on the other hand, there are either no "arms" at all,
or merely short, unbranched, rudimentary arms. The Cystideans are
principally, and indeed nearly exclusively, Silurian fossils;
and though occurring in the Upper Silurian in no small numbers,
they are pre-eminently characteristic of the Llandeilo-Caradoc
period of Lower Silurian time. They commenced their existence,
so far as known, in the Upper Cambrian; and though examples are
not absolutely unknown in later periods, they are pre-eminently
characteristic of the earlier portion of the Palaeozoic epoch.
[Illustration: Fig. 47.--Lower Silurian Crustaceans. a, _Asaphus
tyrannus_, Upper Llandeilo; b. _Ogygia Buchii_, Upper Llandeilo;
c, _Trinucleus concentricus_, Caradoc; d, _Caryocaris Wrightii_,
Arenig (Skiddaw Slates); e, _Beyrichia complicata_, natural size and
enlarged, Upper Llandeilo and Caradoc; f, _Primitia strangulata_,
Caradoc: g. Head-shield of _Calymene Blumenbachii_, var.
_brevicapitata_, Caradoc; h, Head-shield of _Triarthrus Becki_
(Utica Slates), United States: i, Shield of _Leperditia
Canadensis_, var.
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