Upper Silurian series, the double forms (such as C, Fig. 121) being more characteristic of the Lower division, while the single forms run throughout the system.

Their re

Corals abound in some parts of the Silurian seas. mains chiefly occur in the limestones, doubtless because these rocks were formed in comparatively clear water, in which the corals could flourish. But they differed in structure from the familiar reef-building corals of the present day. The great majority of them belonged to the family of the Rugose corals, now only sparingly represented in the waters of the present ocean. As their name denotes, they were particularly marked by their thick rugged

FIG. 122.-Silurian Corals. (a) Rugose Coral (Omphyma turbinatum, ). (b)
Alcyonarian Coral (Heliolites interstinctus, natural size).

walls. Many of them were single independent individuals; some lived together in colonies; while others were sometimes solitary, sometimes gregarious. A typical example of these rugose forms is Omphyma, shown in Fig. 122 (a). Other genera were Cyathaxonia, Cyathophyllum, and Zaphrentis. There were likewise less numerous and more delicate compound forms belonging to what are known as the Tabulate corals (Favosites, Halysites), while another type (Heliolites, Fig. 122, b) represented in ancient times the Alcyonarian corals (Heliopora) of the present time.

Crinoids or stone-lilies played an important part in the earlier seas of the globe. In some regions they lived in such abundance on the sea-floor that their aggregated remains formed solid beds of limestone hundreds of feet thick, and covering thousands of square miles. As their name denotes, crinoids are lily-shaped animals, having a calcareous, jointed flexible stalk fixed to the bottom and supporting at its upper end the body, which is com

posed of calcareous plates furnished with branched calcareous arms (see Figs. 149, 165, 173). It is these hard calcareous parts which have been so abundantly preserved in the fossil state. Remains of crinoids are found in various parts of the Silurian system (Dendrocrinus, Glyptocrinus) chiefly in the limestones, but not in such abundance and variety as in later portions of the Palæozoic formations (compare pp. 265, 277, and Figs. 149, 165, and 173). Allied to the crinoids were the Cystideans, a curious order of echinoderms, with rounded or oval bodies en

FIG. 123.-Silurian Echinoderms. (a) Cystidean (Pseudocrinites quadrifasciatus, natural size). (6) Star-fish (Palæasterina stellata, 3).

closed in calcareous plates, possessing only rudimentary arms, and a comparatively small and short jointed stalk. They first appeared in the Cambrian period (Protocystites), but attained their chief development during Silurian time, thereafter diminishing in numbers. They are thus characteristically Silurian types of life. One of them is represented in Fig. 123 (a). Star-fishes and brittle-stars likewise occur as fossils among the Silurian rocks. These marine creatures, still represented in our present seas, possess hard calcareous plates and spines, which, being imbedded in a tough leathery integument, have not infrequently been preserved in their natural position as fossils. Some of the genera of star-fishes found in the Silurian system are Palæaster, Palæasterina (Fig. 123, b), Palæochoma. Brittle-stars were represented by Protaster.

Throughout great

In the Silurian system are found many tracks and burrows like those of the Cambrian rocks, indicative of the presence of different kinds of sea-worms. thicknesses of strata, indeed, these markings are sometimes the only or chief fossils to be found. Names have been given to the different kinds of burrows (Arenicolites, Scolithus, Lumbricaria, Fig. 124), and of trails (Palæochorda, Palæophycus). There were likewise representatives of the familiar Serpula, which is found so abundantly on the present sea-bottom, encrusting shells

FIG. 124.-Filled-up Burrows or Trails left by

a sea-worm on the bed of the Silurian sea (Lumbricaria antiqua, ).

and stones with a calcareous protecting tube, inside of which the annelide lives. This tube has been preserved in the fossil state

in rocks of all ages.

The Trilobites, which had already appeared in Cambrian time, attained their maximum development during the Silurian period. A few of the primordial or Cambrian types continued to live into this period, but many new genera appeared. In the Lower Silurian series some of the more abundant genera are Asaphus, Ampyx, Ogygia, and Trinucleus; in the Upper Silurian division characteristic genera are Calymene, Phacops, Encrinurus, Illanus, and Homalonotus (Fig. 125). Trilobites continued to flourish, but in gradually diminishing variety, during the Devonian and Carboniferous periods, after which they seem to have died out. They are thus a distinctively Paleozoic type of life, each great division of the Palæozoic rocks being characterised by its own varieties of the type.

Phyllopod crustaceans likewise attained to greater variety during the Silurian period; some of the more frequent genera are Ceratiocaris (Fig. 126), Discinocaris, and Caryocaris. The Phyllopods attained their maximum development during Palæozoic time, but they have continued in existence ever since, and are at present represented by a number of genera, some of which live in the sea, others in fresh water.

The Mollusca are far more abundant and varied in the Silurian than in the Cambrian rocks. Among the more characteristic Silurian genera of Brachiopods are Atrypa, Leptana, Orthis,

Pentamerus, Rhynchonella, and Strophomena (Fig. 127). Among the lamellibranchs we find the Cambrian genera Ctenodonta and Modiolopsis, with new forms such as Orthonota (Fig. 128), Cleidophorus and Ambonychia.

FIG. 125.-Lower and Upper Silurian Trilobites. (a) Asaphus tyrannus (3); (b) Ogygia Buchii (3); (c) Illanus barriensis (3); (d) Trinucleus concentricus (natural size); (e) Homalonotus delphinocephalus (3).

The Gasteropods played an important part in the fauna of the Silurian sea, for upwards of 1300 species of them have been found in Silurian rocks. Among the more frequent genera are

Bellerophon (Fig. 129), Ophileta, Holopea, Murchisonia, Platyschisma.

Numerous representatives of the chambered cephalopods have been found in the Silurian rocks, especially in the upper division. Among the more frequent genera are Orthoceras (straight, Fig. 130 a), Cyrtoceras (curved), Ascoceras (globular or pear-shaped), Lituites (coiled, Fig. 130 b), and also Nautilus, a genus which has persisted through the greater part of geological time to the present day, and now remains the only

representative of the chambered cepha- FIG. 126.-Silurian Phyllopod lopods formerly so abundant.

Crustacean (Ceratiocaris

Remains of Fishes detected in the papilio).

Upper Silurian rocks are the earliest traces of vertebrate life yet known. They consist partly of plates which are regarded

FIG. 127. Silurian Brachiopods. (a) Atrypa reticularis (natural size), Caradoc beds to Lower Devonian; (b) Orthis actonia (natural size); (c) Rhynchonella borealis (natural size); (d) Pentamerus galeatus (natural size).

as portions of the bony covering of certain placoderms or bone-plated forms (Pteraspis, Cephalaspis, Auchenaspis); partly