ed 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. _Josephiana_, of the natural size, Trenton
Limestone, Canada; j, The same, viewed from the front. (After
Salter, M'Coy, Rupert Jones, and Dana.)]

The Ringed Worms (_Annelides_) are abundantly represented in the
Lower Silurian, but principally by tracks and burrows similar
in essential respects to those which occur so commonly in the
Cambrian formation, and calling for no special comment. Much more
important are the _Articulate_ animals, represented as heretofore,
wholly by the remains of the aquatic group of the _Crustaceans_.
Amongst these are numerous little bivalved forms--such as species
of _Primitia_ (fig. 47, f), _Beyrichia_ (fig. 47, e), and
_Leperditia_ (fig. 47, i and j). Most of these are very small,
varying from the size of a pin's head up to that of a hemp seed;
but they are sometimes as large as a small bean (fig. 47, i),
and they are commonly found in myriads together in the rock. As
before said, they belong to the same great group as the living
Water-fleas (_Ostracoda_). Besides these, we find the pod-shaped
head-shields of the shrimp-like Phyllopods--such as _Caryocaris_
(fig. 47, d) and _Ceratiocaris_. More important, however, than
any of these are the _Trilobites_, which may be considered as
attaining their maximum development in the Low