The twelfth family, Cardiadæ, contains the familiar cockles--Cardium--which is derived from καρδὶα, _a heart_, which they are supposed to resemble in form, are amongst the most widely-distributed of shells. The shell is convex, as we see in _C. hians_ (Fig. 144), somewhat heart-shaped, equivalved, the edges dentate or corrugated, the hinge furnished with four teeth upon each valve. The accessary ornaments vary with the species, some being smooth, as in _Cardium Greenlandicum_, Chemnitz (Fig. 145); others, and by far the greater number, are furnished with regular sides, generally obtuse, sometimes in ridges diverging from the point and armed with straight or curved spines, arranged in the oddest manner, as in _Cardium aculeatum_ (Fig. 146). [Illustration: PLATE XVIII.--Venus and Cytherea. I. Venus plicata. (Gmel.) II. Venus puerpera. (Linn.) III. Venus reticulata. (Linn.) IV. Venus Gnidia. (Broderip.) V. Cytherea zonaria. (Lamarck.) VI. Cytherea petechialis. (Lamarck.) VII. Cytherea maculata. (Linn.) ] In _C. hians_ (Fig. 144), the mantle has a large opening in front, fringed anteriorly with papillæ in the form of tentacula; the inhabitant of the shell has a very large foot, with a bend or knee near the middle; its mouth is transverse and funnel-shaped, and furnished with a triangular appendage. One of the peculiarities in the organization of these molluscs is its direct connection with their mode of life. In short, these molluscs, which most commonly live on the sea shore, and bury themselves in the sand to the depth of four or five inches, are enabled to breathe, to draw water for their nourishment, and also to throw off the products of digestion, by having the mantle prolonged into two tubes, the orifices of which reach to the surface of the soil. By means of these feet and an extremely curious organ of locomotion, the Cardiums can at will issue from their holes and re-enter them. The fishermen of the shore easily recognize the presence of these animals by the little jets of water which they throw up through the sands. [Illustration: Fig. 144. Cardium hians (Brocchi).] [Illustration: Fig. 145. Cardium Greenlandicum (Chemnitz).] [Illustration: Fig. 146. Cardium aculeatum (Linnæus).] [Illustration: Fig. 147. Cardium edulis (Linnæus).] These molluscs are found in every sea on the globe, and under all latitudes. Many of them belong to our own and the French coasts, where they are eagerly sought for by collectors, as well as for food. The flesh of the animal, however, is coriaceous, and little esteemed. The species most common on the littoral of the Atlantic is _Cardium edulis_ (Fig. 147), its white or fawn-coloured shell being hollowed out into six and twenty furrows, forming so many corrugated ripples on its side. [Illustration: Fig. 148. Cardium costatum (Linnæus).]