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. 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 Lower Silurian.  The huge Paradoxides of the Cambrian have now disappeared, and with them almost all the principal and characteristic “primordial” genera, save Olenus and Agnostus.  In their