In Russia, also, Messrs. Murchison and De Verneuil have shown (1840) that the "Old Red" group occupies a wide area south from St. Petersburg. It was formerly supposed to be the New Red Sandstone, on account of its saliferous and gypseous beds; but it is now proved to be the Old Red by containing ichthyolites of genera which characterize this group in the British Isles, as, for example, Holoptychius, Coccosteus, Diplopterus, &c.[349-A], associated with mollusca found in the Devonian of Western Europe. Among the fish are also many species of sharks of the Cestraciont division, a fact worthy of notice, because the squaloid fishes of the present day offer the highest organization of the brain and of the generative organs, and make, in these respects, the nearest approach to the higher vertebrate classes.
Devonian Strata in the United States.
The position of this formation between the carboniferous rocks of Pennsylvania and Ohio, is pointed out in the section, [fig. 379.] [p. 327.], and it is a remark of M. de Verneuil that in no European country is there so complete and uninterrupted a development of the Devonian system as in North America. At the falls of the Ohio, at Louisville, in Kentucky, there is a grand display of one of the limestones of this period, resembling a modern coral reef. A wide extent of surface is exposed in a series of horizontal ledges, at all seasons, when the water is not high; and the softer parts of the stone having decomposed and wasted away, the harder calcareous corals stand out in relief, and many of them send out branches from their erect stems precisely as if they were living. Among other species I observed large masses, not less than 5 feet in diameter, of Favosites gothlandica, with its beautiful honeycomb structure well displayed, and, by the side of it, the Favistella, combining a similar honeycombed form with the star of the Astrea. There was also the cup-shaped Cyathophyllum, and the delicate network of the Fenestella, and that elegant and well-known European species of fossil, called "the chain coral," Catenipora escharoides, with a profusion of others (see [fig. 423.] [p. 355.]). These coralline forms were mingled with the joints, stems, and occasionally the heads, of lily encrinites. Although hundreds of fine specimens have been detached from these rocks, to enrich the museums of Europe and America, another crop is constantly working its way out, under the action of the stream, and of the sun and rain, in the warm season when the channel is laid dry. The waters of the Ohio, when I visited the spot in April, 1846, were more than 40 feet below their highest level, and 20 feet above their lowest, so that large spaces of bare rock were exposed to view.[349-B]
Devonian Flora.
With the exception of the fucoids above mentioned ([p. 344.]), but little is known with certainty of the plants of the Devonian group. Those found in the department of La Sarthe in France, and in various parts of Brittany, formerly referred to the Devonian era, have been shown (in 1850), by M. de Verneuil, to belong to the carboniferous series. The same may be said of the species of Lepidodendron, Knorria, Calamite, Sagenaria, and other genera recently figured (1850), by Mr. F. A. Römer, from the formation called "Greywacké à Posodonomyes" in the Hartz.[350-A] They are accompanied by Goniatites reticulatus Phillips, G. intercostatus Phil., and other mountain limestone species, and had been previously assigned to the oldest part of the carboniferous series by Messrs. Murchison and Sedgwick.
If hereafter we should become well acquainted with the land plants of the Devonian era, we may confidently expect that nearly all of them will agree generically with those of the carboniferous period, but the species will be as different as are the Devonian vertebrate and invertebrate animals from the fossil species of the Coal.
CHAPTER XXVII.
SILURIAN GROUP.
Silurian strata formerly called transition — Term grauwacké — Subdivisions of Upper and Lower Silurian — Ludlow formation and fossils — Wenlock formation, corals and shells — Caradoc and Llandeilo beds — Graptolites — Lingula — Trilobites — Cystideæ — Vast thickness of Silurian strata in North Wales — Unconformability of Caradoc sandstone — Silurian strata of the United States — Amount of specific agreement of fossils with those of Europe — Great number of brachiopods — Deep-sea origin of Silurian strata — Absence of fluviatile formations — Mineral character of the most ancient fossiliferous rocks.