The true Cockles, some few species of which are known to almost every one, constitute the family _Cardiadæ_, so called on account of the cordate or heart-shaped form of the shell as viewed from the anterior or posterior side. The shell is regular, or nearly so, and the valves, which are equal, are ornamented with prominent rays that run from the umbones to the margin. The ligament is short, strong and prominent, and the valves fit closely by the interlocking of their crenulated margins, or gape slightly on the posterior side. There are two central teeth in each valve, and a long lateral tooth both on the anterior and posterior sides. The mantle lobes are open in front, with the margins plaited, and the siphons, which are usually short, are provided with a number of little tentacles. The foot is large and powerful, and is usually curved into the form of a sickle. Although the general nature of the common edible cockle (_Cardium edule_) is so well known even to the inhabitants of inland towns that a description may seem out of place here, yet it is possible that but few of our readers have ever taken the trouble to place the animal in a vessel of sea water, either obtained direct from the sea or artificially prepared, for the purpose of studying its movements or other habits; and it will be well to remember that this and several other species of edible molluscs which reach our towns alive may be very conveniently studied at home, and often at times and seasons when work at the sea-side is undesirable or impossible. [Illustration: FIG. 146.--1. _Cardium pygmæum._ 2. _Cardium fasciatum._ 3. _Cardium rusticum_] The edible species referred to lives in banks of sand or mud, buried just below the surface, and frequently in spots that are exposed for several hours between the tides. They are usually obtained by means of a rake similar to that used in our gardens. [Illustration: FIG. 147.--_Cardium aculeatum_] On the coasts of Devon and Cornwall we find a much larger species, also valued as an article of diet, and known locally as the Prickly Cockle (_C. aculeatum_). Its shell is beautifully formed, the rays being very prominent, each bearing a number of calcareous spines arranged in a single row. We give an illustration of this species, together with two sketches to show the nature of the teeth of the shell. In addition to the two species named, we have the red-footed, _C. rusticum_, which can suddenly turn itself over by the action of its powerful pedal organ; the Banded Cockle (_C. fasciatum_), a very small species distinguished by the brown bands of the shell; and a still smaller one (_C. pygmæum_), with a triangular shell, occurring on the Dorset and Devon coasts (fig. 146).