A Pictorial Atlas of Fossil Remains, consisting of coloured illustrations selected from Parkinson's "Organic remains of a former world," and Artis's "Antediluvian phytology."

by Gideon Algernon Mantell

[V.] Fossil Cephalopoda, Nautilus, Ammonite, &c.—The fossil remains of the molluscous animals, named Cephalopoda, from their organs of prehension being arranged around the head or upper part of the body, are the most ancient, numerous, and interesting, of this class of animated nature in the mineral kingdom. These relics are among the most varied and striking of the extinct beings that occur in the sedimentary strata, from the most ancient secondary formations, to the most recent tertiary. The living species are but a feeble representation of the countless myriads which must have swarmed in the ancient seas.

The animal of the Cephalopods is composed of a body, which is either enclosed in a shell, as in the Nautilus, or contains a calcareous osselet or support, as in the Sepia or Cuttle-fish; it has a distinct head, and eyes as perfect as in the vertebrated animals, with complicated organs of hearing, and a powerful masticating apparatus, surrounded by arms or tentacula. Below the head there is a tube which acts as a locomotive instrument, to propel the animal backwards, by the forcible ejection of the water that has served the purpose of respiration, and which can be ejected with considerable force by the contraction of the body.

Their fossil remains consist of the external shell and the internal osselet; and in the naked tribes, of the soft parts of the body, the ink-bag, &c., as noticed in the account of the Belemnite and Belemnoteuthis.

The shell varies exceedingly in the different genera. In the group characterised by smooth septa, and a medial or submedial siphuncle, as the Nautilus, the earliest or most ancient type is straight, as in the Orthoceras ([Plate LVIII.] fig. 14) of the palæozoic formations; the intermediate forms present various modifications of the spiral, and terminate in the completely discoidal shell of the living genus; while the other group, that with sinuous or foliated septa and a dorsal siphuncle, commences in a discoidal type—the Ammonite, which gradually passes through the various modifications of Crioceras, Scaphites ([Plate LXI.] fig. 10), Hamites ([Plate LXI.] fig. 3), Turrilites ([Plate LXI.] fig. 12), &c.; and finally becomes extinct in the straight Baculites ([Plate LX.] fig. 2).

In argillaceous strata, as the Kimmeridge and Oxford clay, London clay, &c., the shells of Cephalopoda are oftentimes beautifully preserved; the chambers are frequently filled with the solid matrix, but in many instances these cavities are lined either with brilliant pyrites or spar. Stony or sparry casts of the cells or chambers, the shell having perished, are another common state in which vestiges of these animals occur. Sometimes the cast of each chamber is isolated, so as to present a series from the innermost to the outermost cell. Sections of those casts, in which the chambers are filled up with spar, constitute specimens of great beauty and interest. The so-called snake-stones are, of course, mere casts of Ammonites;[104] those of Whitby, from the lias limestone, are well known to every collector; the casts of a very large species are common in the oolite, especially at Swindon, in Wiltshire, and in the neighbourhood of Bath.

[VI.] The Carboniferous Deposits, or Coal Measures.—The various deposits of Coal have manifestly been formed under different local circumstances. Some have been peat-bogs, to which repeated additions have been made by successive subsidences of the land; others have been deposited at the bottom of lakes and rivers, and these are associated with remains of fresh-water shells and Crustacea; others have accumulated in the abyss of the ocean, having been formed by the drifting and engulfing of immense rafts of trees and other vegetable matter, like those of the Mississippi; others in inland seas, the successive layers of vegetables having been supplied by periodical land-floods. There can be no doubt that coal has been, and may be, produced under all these conditions; and at different periods, and in various localities, all these causes may have been in operation. But the great series of ancient coal-formations present a remarkable uniformity of character, not only throughout Europe, but also in America and other parts of the world. A coal-field (as a group of strata of this kind is commonly termed), is generally composed of a series of layers of coal, clay, shale, and sand, of variable thicknesses, based on grit or limestone, abounding in marine shells and corals; and the most remarkable phenomenon is the constant presence beneath every bed of coal, of a thick stratum of earthy clay, and of a layer of shale or slaty clay above it. One of the series of triple deposits of which a coal-field consists, presents therefore the following characters:—

1. Under-clay; the lowermost stratum. This is a tough argillaceous earth or clay, which on drying becomes of a grey colour, and very friable; it is occasionally black, from an intermixture of carbonaceous matter. This bed almost invariably contains an abundance of Stigmariæ (see Plates XXII. XXIII.), of considerable length, with their rootlets attached, and which extend in every direction through the clay (as shown in the figures 1, 2, 6, pp. 199, 201). These roots commonly lie parallel with the planes of the stratum, and nearer to the top than to the bottom.

2. Coal.—A carbonized mass, in which the external forms of the plants and trees composing it are obliterated, but the internal structure, in many instances, remains. Large trunks, and stems, and leaves, are rarely found in it.

3. The Roof, or upper bed.—This consists of slaty clay, abounding in leaves, trunks and branches, fruit, &c.; it includes layers and nodules of ironstone, inclosing leaves, insects, Crustacea, &c. In some localities beds of fresh-water shells, in others of marine shells, are intercalated with the shale; finely laminated clay, micaceous sand, grit, and pebbles of limestone, sandstone, &c. are also often interstratified. The principal illustrative specimens of the leaves, fruit, &c. (as those in [Plate XXX.] to [Plate XXXIV.]) are found in this bed.

Thus an uninterrupted series of strata, in which triple deposits of this kind are repeated, (often thirty or forty times, and through a thickness of several thousand feet,) constitutes the predominant character of the ancient coal formations wherever they have been explored. The difficulties attending a satisfactory solution of this problem, are fully stated in the last edition of my Wonders of Geology (Vol. ii. Lecture vii.), and to that work I must refer the reader for a more extended consideration of this highly interesting subject.