[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
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.) 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 Palæozoic epoch.
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
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.) 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 place we have a great number of new forms—some of them, like the great Asaphus tyrannus of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera—Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illœnus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis, and Homalonotus, a few of them passing upwards under new forms into the Upper Silurian.
Coming next to the Mollusca, we find the group of the Sea-mosses and Sea-mats (Polyzoa) represented now by quite a number of forms. Amongst these are examples of the true Lace-corals (Retepora and Fenestella), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to shells and corals (Hippothoa, Alecto, &c.); but perhaps the most characteristic forms belong to the genus Ptilodictya (figs. 48 and 49). In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides. Each of these little chambers or "cells" was originally tenanted by a minute animal, and the whole thus constituted a compound organism or colony.
The Lamp-shells or Brachiopods are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of "Age of Brachiopods" has with justice been applied to the Silurian period as a whole. It would be impossible here to enter into details as to the many
