[381] The collector may obtain specimens, and polished slabs of these limestones, of Mr. Martin, mason, Lewes, Sussex.
Lign. 134.
Polished Slab of Purbeck Marble.
Limnæa. [Lign. 133, fig. 2.]—Several species of these fresh-water mollusks inhabit our lakes and ponds, and may be known by their pointed spire, elongated oval body, and delicate thin shell: on the inner lip of the aperture there is an oblique fold. Fossil shells of this genus are found with Paludinæ in the fresh-water tertiary deposits. Headon Hill and other localities in the Isle of Wight abound in these shells; and in the limestone of Calbourn beautiful casts are very numerous. The Paris basin yields several species; and there are six species in the Isle of Wight Tertiary; I have not observed any decided examples in the Wealden. In the sands and clays the shells are well preserved; in the limestones the casts only remain. Shells of another genus of fresh-water spiral univalves, termed Bulimus (Ly. p. 30), are found associated with the above. A large species (B. ellipticus, Min. Conch. tab. 337), occurs in the limestone at Binstead, near Ryde, and at Calbourn; I have collected specimens two inches long from the former locality; they are generally in the state of casts, with a white friable coating of the shell.[382]
[382] A very large species of Limnæa from Bavaria (labelled L. maxima) is in the British Museum. It is a cast six and a half inches long, and is placed with the recent shells. Prof. E. Forbes has discovered a Limneïd (Physa) in the Purbeck strata.
Planorbis. Ly. p. 29. Wond. p. 400.—The shells of this genus are also numerous in our rivers and lakes, and may be distinguished by their discoidal form, the shell being coiled up in a nearly vertical plane. There are about twenty living species; and sixteen are enumerated as fossil in the British tertiary; five occur in the Isle of Wight basin, in the localities of the fresh-water genera already mentioned; Headon Hill, in particular, yields shells of this genus in great abundance and perfection.
Melanopsis. Ly. p. 29.—These are spiral univalves, the appearance of which will be better understood by the figures, than by any description. I allude to this genus because a small species is very numerous, with the other fresh-water shells, at Headon Hill; and two or more species are found in the argillaceous strata of the Wealden (see Geol. S. E. p. 249, and [Lign. 132]).
FOSSIL MARINE UNIVALVES.
Marine Univalves.—Of the fossil marine Gasteropoda there are no less than eighty genera in the strata of the British Islands, and the species amount to several hundreds. To distinguish the species and genera, reference must, of course, be made to works expressly devoted to fossil conchology, as Sowerby's Mineral Conchology, and Genera of Fossil Shells; or to the works of French authors, particularly those of Lamarck, edited by M. Deshayes, and of M. Blainville. The Penny Cyclopedia contains admirable notices of fossil shells, under the respective heads of the classes, orders, and genera, of the recent Mollusca.