Crinoids Cyathocrinus, Crotalocrinus, Taxocrinus, Periecho crinus.
Crustaceans Eurypterus, Pterygotus, Slimonia.
Cephalopods Orthoceras, Gomphoceras, Trochoceras, Phragmoceras, A scoceras.
Lamellibranchs Orthonota, Cardiola, Aviculopecten, Pterinaea, Modio lopsis, Grammysia.
In the Graptolitic facies Monograpti predominate throughout; in the earlier beds they have either simple cells or cells with a slight degree of sinuosity, but they then tend to show a develop ment into a lobe or hook on the one hand, or to isolation of the cells from each other on the other ; thus the lobed cell and the isolated cell are characteristic of the Valentian, the hooked cell of the lower part of the Salopian, and the return to simple form the upper part of the Salopian. In general, graptolites are rare above the Salopian, but some have been recorded from Bohemia from beds of Downtonian age.
Europe.—With the exception of those occurring in E. Scan dinavia, Estonia, Livonia and Russia, the beds in Europe have been in general highly folded by earth movements, but even so may be grouped according to their facies of deposit; they seem to have occupied much the same areas as those of the Ordovician, and it is clear from the study of the rocks accumulating through out the period in the N.W. geosyncline, that the stresses were in operation that were eventually to culminate in the elevation of the Caledonian-Scandinavian mountain system, for the shore lines of the gulf are continually shifting their position, and as a result intra-formational conglomerates are frequently found, and there is a marked tendency for succeeding beds to overlap; moreover, oscillations of the shore line gave origin to a widespread con tinental shelf with its island festoon affording ideal conditions for the reef-building by corals and other organisms, or for the forma tion of shell-banks made up largely of the remains of one class of organism. Outside the geosyncline also clean sea limestone reefs or graptolite shales are of widespread occurrence. The island of Gotland may be taken as typifying the former, the development in Skane or Bohemia as characteristic of the latter facies of deposit.
The British Isles.—The general instability of the shore lines of the geosyncline is admirably illustrated by the rock succession in S. Scotland on the one hand and the Welsh borderland on the other; in S. Scotland the shore line moved more or less steadily towards the S.E., so that deeper water beds are continually being overlaid by those of shallower water type. In the Welsh borderland the oscillations were far more irregular; sometimes the sea trans gressed with overlap and unconformity upon the eastern shore line, at others there was a retrogressive movement when the shore line advanced so that there is great variety in different places, not only in the sediments found, but also in the complete ness of the succession. One fact remains constant, namely, that when far enough away from the shore line, these variable deposits all tend to pass into graptolite shales until the area of the gulf has become so shallow that the shallow water facies occupies the whole.
Scandinavia.—Skane alone possesses a complete series of beds of Silurian age ; for the greater part these belong to the graptolite shale facies, but include shallower water beds towards the top : over much of the remainder of the country the strata of this period are found in the table-topped hills capped by dolerite in which only a part of the succession is found.
America.—The Silurian rocks of America are not exposed so extensively as were those of Ordovician age; at present little is known about their character in the west, the best known succes sions being found in the east and centre, though they are also known in the north; in the eastern region the succession is of interest, as a volcanic facies occurs in addition to those of shallow and deeper water type and in many respects, apart from the volcanic rocks, the succession resembles that of the British Isles; the central region contains the classic development exposed in the Niagaran Gorge (see table) ; the rocks are thickest in the Appa lachian region, and thin away towards the interior where the series tends to be incomplete as the result of unconformity or overlap. There is a very general absence of rocks of Lower Valentian age. In the north the development, which includes islands in the Arctic archipelago, consists very largely of coral limestones.
Australia.—Strata of Silurian age appear to be restricted wholly to the east part of the continent, being found only in Queensland, New South Wales, Victoria and Tasmania, where they have been considered as originating in Schuchert's "Tasmanian geosyncline." They are mainly of interest as affording evidence of submarine volcanic activity during the period, this being most conspicuously displayed in New South Wales ; apart from this, records indicate the occurrence of the common facies of deposition. In Tasmania the rocks are mainly of shallow water type, and in Victoria where they have been considerably affected by earth movement, they are distinguished by the development of fringing reefs of coral lime stone. Graptolite shales with characteristic Monograpti have also been recorded.
Isles: B. N. Peach, J. Horne and J. J. H. Teall, The Silurian Rocks of Britain, Vol. I. (Scotland) in the Memrs. of the Geol. Survey of the U.K. (H.M. Stationery Office, 1899) ; Articles in the Quarterly Jrnl. of the Geol. Soc. by H. A. Nicholson and J. E. Marr, vol. 44 (1888), G. L. Elles, 56 (1900), E. M. R. Wood, 56 (1900), G. L. Elles and I. L. Slater, 62 (1906), 0. T. Jones, 65 (1909), 77 (1921), and 81 (1925) ; J. E. Marr, in the Geol. Mag. N.S., Decade III., Vol. IX. (1892) ; L. D. Stamp, in the Geol. Mag., Vol. 6o (1923). Scandinavia: J. Kiaer, Des Obersilur im Kristianiagebez'te, Videnskapsselskapets Skriften I. Mat-Naturv. (Christiania, 5908) ; J. C. Moberg, "Historical-Stratigraphical Review of the Silurian of Sweden" in Sveriges geologiske Undersokning, Arsbok Series C. No. 229 (Stockholm, 1911). America: B. Willis, Index to the Stratigraphy of North America (U.S. Geol. Soc., 1912). Australia: T. W. E. David, Geology of the Commonwealth, Federal Handbook of Australia (Melbourne, 1914). Bohemia: J. Perner and 0. Kodym, in the American. Jrnl. of Science, Ser. 5., Vol. IV. (1922) .