Buccinum, of which the common Whelk is an example.—Fusus, [Lign. 133, fig. 4.] Wond. p. 244.—Pleurotoma, Ly. p. 31. Wond. p. 244.—Cerithium, [Lign. 133, fig. 3]. Wond. p. 244.—Ancilla, Wond. p. 244. Ly. p. 31.—Voluta, Ly. p. 202, fig. 180.—Murex, Ly. p. 164.—Rostellaria, Ly. p. 201.—To the eight genera here enumerated a very large number of the marine simple univalve shells belong; and they are principally found in Tertiary strata.
The animals of these shells are characterized by their respiratory organs, which are formed of one or two pectiniform gills, with a tube or siphon more or less elongated, for the free admission of sea-water to the branchial apparatus. This organization is indicated in the shell, either by a notch, or by a prolonged tubular canal. All the species are, with scarcely any exceptions, inhabitants of the sea, and carnivorous.
I have selected for illustration of the genus Fusus, a celebrated shell of the Crag, known among collectors as the "Esssex reversed Whelk," [Lign. 133, fig. 4]; the spire is twisted in the opposite direction to the usual mode, and the mouth is consequently to the left of the observer; the same species occurs with the spire in the common direction. The shells of the genus Pleurotoma are distinguished by an incision, or notch, in the side of the right or outer lip; and those of Cerithium, by the form of the mouth, see [Lign. 133, fig. 3]. The latter is a very numerous genus, and more than two hundred fossil species are enumerated; it contains many elegant forms. The Tertiary strata at Grignon are particularly rich in these fossils; the shells are of a pearly whiteness, and as perfect as when recent. Some Cerithia are of considerable size; the C. giganteum is from ten to fourteen inches in length. The genus Potamides comprehends shells closely resembling the Cerithia in form, but which are inhabitants of fresh-water.[383] This is an instance of the difficulty which sometimes exists of arriving at certain conclusions as to the habits of the mollusks, from their testaceous coverings alone.
[383] Mr. Woodward informs me that they can only be distinguished when fossil, by the absence of varices, or "periodic mouths." The recent species are known to be inhabitants of fresh-water, by their dark epidermis, corroded points, and horny multi-spiral opercula.
The Plastic Clay beds at Castle Hill, Newhaven, and in the vicinity of Woolwich, abound in two species of shells, which were originally described by Mr. Sowerby, as Cerithia (viz. C. funatum and C. melanoides),[384] but are now referred to the fresh-water genus, Melania; by some conchologists to Potamides. At Castle Hill they are accompanied by fresh-water bivalves, and leaves of dicotyledonous plants.
[384] Foss. South Downs, tab. xvii. figs. 3, 4.
Of the genus Rostellaria, there is a remarkable species in the London Clay, called R. macroptera, from the large wing-like expansion of its outer lip, in adult specimens; see Ly. p. 201. An elegant Rostellaria is found in the Galt, at Folkstone,[385] (Foss. South D. tab. xix. figs. 12, 14,) and other localities; and also in the Chalk Marl.
[385] "This shell belongs to the recent genus, Aporrhaïs, and is related to Cerithium, not to Strombus."—Mr. Woodward.