The Medals of Creation, Volumes 1 and 2 / First Lessons in Geology and the Study of Organic Remains

by Gideon Algernon Mantell

The shells of the Inocerami, like those of the oyster, and other living mollusca, were exposed to the attacks of some parasite, and perhaps of some Annelid, as the Nereis. The shells are often cellular from this cause, and the cavities are found either hollow, or filled with chalk, or, as in the example [Lign. 130], with flint. In the latter case, upon the decomposition of the shell, the siliceous casts remain in relief on the surface of the flint, as in [Lign. 130], b. Such specimens are common in the broken flints of the South Downs, and in the shingle on the sea-shore of chalk districts; and their origin would be difficult to understand without this explanation.[365]

[365] The Rev. W. Conybeare first ascertained the origin of these fossils, and figured and described them in an elegant Memoir, published in Geol. Trans, vol. ii. first series. Mr. Morris proposes the name of Clionites for the fossil bodies derived from the depredations of the Cliona on the Inocerami and other shells. See Annals Nat. Hist. 1851, and my Pictorial Atlas of Organic Remains.

Avicula. Lyell, p. 274.—Above fifty species of this genus of shells have been found in the British strata; their general character will be readily understood by reference to the pearl-oyster, (Avicula margaritifera,) which is so largely imported for the manufacture of mother-of-pearl ornaments. A remarkable species is found in the Lias, called, from the great disproportion in the size of the shells, Avicula inæquivalvis, (Lyell, p. 274.) The recent species are inhabitants of warm climates.

Our limits will not admit of further notice of the Monomyaria, and we proceed to the second division of the plated-gilled mollusca.

Dimyaria: Bivalve Shells, with two muscular imprints.

The conchifera, or bivalve shells, of this group, found fossil, are more than double in number those of the preceding; nearly eight hundred species are known in the rocks of Great Britain, of which by far the greater number is marine. But we must restrict our notice of this division to a few genera, that more space may be devoted to that important class, the Cephalopodous Mollusca.

The Cardium, Venus, and Mussel shells, are familiar examples of the Dimyaria. The conglomerates, now forming in the British Channel, from accumulations of the recent species of Cockle (C. edule), have been previously noticed; see [Lign. 124], [p. 386]. In the strata of England there are upwards of thirty species: the Crag contains several, particularly a large and delicate shell, the Cardium Parkinsoni (Min. Conch. tab. 49). Others are peculiar to the London clay, as the Cardium semigranulatum, a beautiful shell, having the surface smooth, except on the posterior side, which is covered with strong ridges, beset with minute granules; it is found in many localities (Min. Conch. tab. 144). Among the silicified shells of the Shanklin sand of Devonshire, an elegant Cardium, C. Hillanum, (Min. Conch. tab. 14,) occurs. But one species is known in the formations below the Lias: the Cardium striatum, (Murch. Sil. Syst. tab. 6, fig. 2,) found in the Aymestry limestone.

VENERICARDIA. PECTUNCULUS.

Venericardia. Ly. p. 199.—These shells are abundant in the tertiary strata; one large species, V. planicosta, (Ly. p. 199, fig. 171,) is found in immense quantities in the clay and sand at Bracklesham Bay, in Sussex, from the young to the adult state; some examples are very large, and perfect. In the sand at Grignon, near Paris, the same shell is abundant, possessing the usual white and delicate aspect of the fossils of that celebrated locality of the Calcaire grossier. Only one species has been noticed in the British secondary strata.

Pectunculus. Wond. p. 244, fig. 8.—In the London clay at Bracklesham Bay, Highgate, Hordwell Cliff, and in the arenaceous limestone of Bognor rocks, an immense number of the bivalve shells, called Pectunculi (little pectens), occur. Some of the French marine tertiary strata also abound in the same, and other species of this genus. In the above-mentioned Sussex localities, these shells are so numerous, as to be the most frequent fossils that come under the notice of the collector. They are readily known from their associates by their rounded equivalve shells, and the single arched row of teeth along the hinge, resembling the common Arca.[366] (See Min. Conch. tab. 27). At Plumstead, near Woolwich, a smaller species is found; and also occasionally with the oysters at Bromley.