Our knowledge concerning the land vegetation of the Silurian is not much more definite than concerning that of the Ordovician. Most of the remains referred to land plants are of disputable character; the best authenticated is a fern (Neuro-pteris) from the Silurian of France.
Spongida are still common. Well-known forms are Astylo-spongia and Astrceospongia (PL VIII, Fig. i); both are almost restricted to Tennessee.
The Graptolites have greatly diminished, especially the branching forms and those with two or more rows of cells. Those that persist are, for the most part, straight and simple. The Hydroid Corals, on the other hand, such as Heliolites, Stromatopora, etc., become important elements of marine life and in the formation of the reefs. The true Corals likewise increase largely, and play a more important role than in the preceding period. The increase is principally in the enlarged number of species belonging to much the same genera. Favosites (VIII, 2) is a characteristic new genus, and Halysites (VIII, 3), the chain coral, Syringopora, and others are much commoner than before.
In this group we observe a great expansion of the Cystoidea, which are very abundant in the Chicago limestone (Niagara) and the Manlius of Maryland and West Virginia. Holocystites and Caryocrinus (VIII, 4, 5) are typical of the fauna. There is also a marked increase of the Crinoids ; Eucalyptocrinus (VIII, 6) is a good example. Starfishes also have grown more abundant. The Blastoidea, which originated in the Ordovician and became extinct at the end of the Palaeozoic era, remain rare in the Silurian and Devonian, first becoming important in the Carboniferous. The Echinoids, or sea-urchins, which were commoner than before, have no arms, but a closed spheroidal or discoidal test, made up of calcareous plates, which in all the modern sea-urchins are arranged in just twenty vertical rows, and are closely fitted together by their edges, like a mosaic pavement. In the Palaeozoic sea-urchins the number of rows of plates is either more or less than twenty; in some of the Silurian genera the plates are loosely fitted, and slightly overlapping, like fish-scales.
The Echinoderms are rare, and belong to the Cystoids, a very primitive grade of the type.
The presence of marine worms is abundantly indicated by tubes, tracks, and borings in the sands which have now consolidated into hard rocks. Hyolithes, a worm-tube (I, 18), is very common.
Among the Crustacea the Trilobites are still numerous, though decidedly less so than they were in the Ordovi-cian; they represent, for the most part, new species of genera which have survived from the preceding period. The commonest genera are Calymmene, Illcenus, Dalmanites Lichas (VIII, 19), Phacops, Proetus, Encrinurus, etc. Especially characteristic are the genera Staurocephalus (VIII, 20) and Deiphon (VIII, 21) and the large number of spiny forms. Eurypterids continue to increase in numbers and size, though not reaching their maxi-, mum in either respect. In these extraordinary Crustacea the head is quadrate and is followed by a long, tapering body, composed of thirteen movable segments; the last segment is either a pointed spine, as in Eurypterus (see Fig. 262), or a broad tail-fin, as in Pterygotus. Five pairs of appendages are attached to the head, the bases of four of which, on each side of the mouth, form the jaws, as in the existing horseshoe crab. The first pair of appendages are either short and simple (Eurypterus, Stylonurus), or are much elongated, and armed with pincers (Pterygotus). The fifth pair are either very long, or enlarged to serve as swimming paddles.
The first body-segment carries a pair of apron-like appendages, with a narrow median extension, but the other segments have no appendages. The horse-shoe crabs find their most ancient representative in the genus Hemiaspis of the European Silurian. The Eurypterida would appear not to have been marine animals but to have lived in the freshening lagoons of the upper Salina and to have been entombed in the shoal-water calcareous muds of the Water-lime. Other Crustacea are much as in the preceding period.
Fig. 262. - Eurypterus fischeri Eichw., Island of Oesel, Russia. (Schmidt).
Scorpions have been found in the Silurian of Europe and America, and some remains of Insects in the former continent. These animals prove the existence of a contemporaneous land vegetation, and confirm the doubtful evidence of the Ordovician and Silurian plants.
Bryozoa decline, but are still quite abundant, and contribute in an important way to the growth of the coral reefs as well as forming reefs by themselves in the Clinton and Lockport limestones, but they are less important reef-builders than they had been in the Ordovician, chiefly because the massive kinds are so largely replaced by delicate, lace-like colonies.
Brachiopoda continue to be present in multitudes, but with a distinct change in dominant genera from those which were commonest in the Ordovician. Especially characteristic is the increase in the families of the Spiriferidce and Pentameridce, both of which continue prominent in the Devonian. The most important genera are Atrypa, Spirifer (VIII, 9, 10), Pentamerns (VIII, 13), Rhynchotreta (VIII, 12), Leptcena, Strep-tis, and Whitfieldella. A curious family of Inarticulate Brachiopods is the Trimerellidae, several forms of which are found in the coral reef dolomites of the Guelph. Monomerella, a genus of this family, is figured (VIII, 8).
Fig. 263. - Lanarkia sp.; a primitive Ostracoderm. (Traquair.) Restored.
The Bivalves show no very significant changes from the Ordovician, but the Gastropods, especially such forms as Capulus, increase decidedly; other well-represented genera of these shells are Platyostoma, Cydonema, Strophostylus (VIII, 14)) and Trematonotus (VIII, 15). Pteropods are smaller and less numerous than before; a very common form is the little nail-shaped shell, Tentaculites (VIII, 7), which is doubtfully referred to this group, but may belong to the Worms. Among the Cepha-lopods Orthoceratites are less frequent than in the Ordovician, but are more commonly ornamented by rings or longitudinal ridges (VIII, 16), and Endoceras has disappeared, while low tur-reted shells, like Trochoceras (VIII, 17), are characteristic. The shells with curiously contracted mouth openings, like Phragmo-ceras, are more commonly found than in the Ordovician.
The remains of Ostracoderms of a primitive kind have been found in the uppermost Silurian of Scotland; they are of small size and very peculiar appearance, but are related to the genera which were to attain such prominence in the Devonian. Sharks, doubtless of extremely primitive character, existed in the Silurian seas, but very little is known about them.
Fig. 264. - Upper figure, Birkenia etegans Traq., x 3/2. Lower figure, Lasaniusprob-lemattim Traq., enlarged. Both figures restored. (Traquair).