The Common Snail, River Snail, and Periwinkle, are instances of terrestrial, fluviatile, and marine forms. The organs of respiration are situated in the last whorl of the shell; and in some genera the border of the mantle, or integument surrounding the body, is prolonged into a siphon, by which the water is freely admitted, without the head or foot being protruded: in these mollusks the shell has a corresponding channel to receive the siphon, as in the Whelk, or Buccinum, and in the fossil shell [Lign. 133, fig. 4]. The Gasteropoda are generally provided with an operculum, or movable valve, by which the aperture is closed and defended when the animal retreats within its shell. In some species the operculum is a mere horny pellicle; in others it is a solid calcareous plate of considerable relative thickness. These mollusca, as is but too well known of the terrestrial species, consume large quantities of food. Some are herbivorous, and others carnivorous; many prey on living, and others on decaying animal and vegetable substances.[378] As in a fossil state the shells alone remain to afford any clue as to the structure and economy of the originals, characters have been sought for, by which the fluviatile or marine nature, and the carnivorous or herbivorous habits of the living mollusca may be determined. As a general rule, it will be found, that the shells of terrestrial and fresh-water Gasteropoda have the aperture entire, as in the Garden Snail, and in the fossil shell, [Lign. 133, fig. 1]; and that a large proportion of the marine species have the opening notched or channelled, as in the Whelk, and [Lign. 133, figs. 3, 4]; and most of the species with entire apertures are herbivorous. But these inferences must be regarded in a very general sense, and it will require corroborative evidence to establish the marine or fresh-water nature of those fossil shells which do not bear a close analogy to known living species.[379]
[378] "All Gasteropoda commence life under the same form, both of shell and animal, namely, a very simple helicoid shell, and an animal furnished with two ciliated wings or lobes, by which it can swim freely through the fluid in which it is contained. At this stage of existence the animal corresponds to the permanent state of the Pteropod, and the form is alike, whether it be afterwards a shelled or a shell-less species."—Prof. E. Forbes, Edin. Philos. Journal, vol. xxxvi. p. 326.
The well known Tiger Cowry (Cyprcea tigris) in its earliest stage has a minute helicoid (snail-like) shell.
[379] See Ly. p. 30.
The various conditions in which the remains of univalve shells occur in the mineral kingdom have already been so fully explained, that but a few additional remarks on that subject are required (see [p. 382].).
The Gasteropoda are found to progressively diminish in number with the antiquity of the deposits, and it was once supposed that this type of molluscous organization was not contemporaneous with the ancient Cephalopoda. My discovery of several genera associated with Ammonites in the chalk (see Foss. South D. pl. xviii, xix) first tended to invalidate this hypothesis; and the subsequent researches of Dr. Fitton, Professor Phillips, and other geologists have shown that the presence or absence of Gasteropoda in a stratum may generally be ascribed to the circumstance of the deposit having been formed in shallow, or in deep water. Thus when simple univalves largely predominate under circumstances that indicate they were imbedded in their native habitats, it may be safely concluded that the rock is of littoral formation; or, in other words, was deposited in shallow water, near the sea-shore; and, on the contrary, when Nautili, Ammonites, and the shells of other mollusca known to live in deep waters abound in a formation, it may be presumed that the strata were formed in the tranquil depths of the ocean. The number of described species from the British strata is nearly eight hundred; and these are distributed throughout the sedimentary formations, from the Silurian to the newest Tertiary; the latter containing by far the greater proportion.
FOSSIL FRESH-WATER UNIVALVES.
Fresh-water Univalves.—The fossil shells of Gasteropoda that are undoubtedly fluviatile, comprise but few genera and species, and are confined to those deposits, which, from the corroborative proofs afforded by other organic remains, are unquestionably of fresh-water origin. Such are the intercalated beds of clay and limestone in the London and Paris basins, the Wealden formation, and certain strata in the Carboniferous system. The most numerous specimens are principally referable to the common fluviatile genera, Paludina, Limnæa, Planorbis, and Melanopsis (see Ly. p. 29).
Paludina. Lign. 133, fig. 1. (Wond. p. 401, Ly. p. 29.)—This common river shell is of a conoidal form, and the whorls of the spire, and the aperture, are rounded. Eleven British species are known. In the tertiary fresh-water beds of Headon Hill, at Alum Bay, Paludinæ with the shells perfect, and of a dull white colour, are abundant; and also in the limestone at Shalcombe, in the Isle of Wight, in the state of casts. In both these localities the Paludinæ are associated with other fresh-water shells. But the grand deposit of shells of this genus is the Wealden formation; throughout which there are extensive beds of marble, coarse limestone, and clays, almost wholly composed of Paludinæ, and minute fresh-water Crustaceans, of the genus Cypris, which will be described in a subsequent chapter. The compact paludina-limestone of Sussex, called Petworth or Sussex marble, is principally made up of one species, the P. fluviorum, [Lign. 133, fig. 1], and is an aggregation of Paludinæ, held together by crystallized carbonate of lime; the cavities of the shells, and their interstices, being often filled with white calcareous spar. A polished slab, displaying sections of the enclosed shells, is figured in Wond. p. 402. Upon examining slices of this marble with the microscope, the cavities of the shells are found to contain myriads of the cases of Cyprides.[380] The Wealden limestone of the Isle of Purbeck, [Lign. 134], known as Purbeck marble, is, in like manner, composed of Paludinæ, but of a much smaller species. Both these marbles were in great repute with the architects of the middle ages, and there are but few of our cathedrals and ancient churches which do not still contain examples, either in their columns, monuments, or pavements, of one or both varieties. The polished marble columns of Chichester Cathedral, and those of the Temple Church, in London, are of Purbeck marble; in other words, they are composed of the petrified shells of snails, that lived and died in a river, flowing through a country inhabited by the Iguanodon and other colossal reptiles, all of which have long since become extinct. With the exception of the mussel-band limestone of the Carboniferous system, previously described, these are the only British fresh-water marbles[381] There are four species of Paludina in the Wealden, and four in the Tertiary strata of Hants.
[380] For a particular account of this marble, see Geol. S. E. pp. 182-187.