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. [Sidenote: 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.