CARDIUM, a genus of Bivalve Mollusca belonging to the Ace phalous Lamdlibranchiata. It is the type of the family Cardiadce, and the species are known by the common name of Cockle. The shell is equivalve, more or less cordiform, oblong or transversely ovate, usually inflated, closed or gaping posteriorly, longitudinally ribbed or furrowed in radiating fashion from the prominent beaks, rarely smooth; ribs often scaly or spiny ; margin almost always crenulated. Hinge composed of two oblique primary teeth in each valve and two remote Lateral ones (in certain exotic forms, the teeth become partially or wholly obsolete). The ligament short, external, conspicuous ; pallial impression simple. The animal is suborbicular, tumid, its mantle freely opening in front with plain or less frequently fringed edges, conspicuously fimbriated in the neighbourhood of the two very short slightly-separated siphons, the bronchial one of which is always fringed at the orifice. Foot very large, cylindrical;geniculated. Branchial leaflets unequal, labial palps rather long and triangular. (Forbes and Hanley.) The ahella belonging to this genus aro very widely distributed, and many of them are remarkable for the elegance of their form and colouring. The species are about 200 in number. " We find," say the authors of the ` History of British Molluscs; "the great assemblage of Cockles in the Indian Ocean, a region where about a third of the species are congregated. Around this centre the number of specific forms diminishes, though found in every sea. They are most plentiful everywhere within the tropics, and diminish as we proceed northward and southward; but some of the forms most prolific in individuals and most gregarious in habit are present in cold climates, and make up by abundance for the absence of variety. Of these several are valuable articles of food; and it may be said of all the Cardia that they hold a high rank among Mollusca, both for nutritive qualities and excellence of flavour. The genus contains several remarkable abnormal forms ; some of the most singular are to be found in the Caspian and other relics of the great Aralo-Caspian Sea—the demonstration of which mighty inland ocean is among the finest discoveries of Sir Roderick Murchison.
"The geological distribution of this interesting group corresponds in extent with the geographicaL Even in Palaeozoic Strata we find the fossilised remains of Mollusks closely allied if not belonging to Cardjurn. In the Secondary Rocks, even in their oldest members, well-marked forms of Cardium are not unfrequent, often singularly eimilating those of existing times. During the later part of the
Secondary epoch and the beginning of the Tertiary a group of half ribbed cockles seemed to have been developed at the expense of ordinary forms, and to have dwindled away as they came near our own epoch, when but two or three allies of them are found." (Vol. iL p. 3, 4.) Cockles inhabit all parts of the ocean. Some species are constantly met with between high and low water marks, and they have presented themselves from the deepest sea-beds. Each species has however a very definite range. They lie buried in sand or sandy mud, often occurring in prodigious quantities. According to the researches of Dr. W. B. Carpenter, the shell of the genus Cardium has a very definite elementary structure. Externally it presents a tubular structure, but internally there is little development of organic structure.
C. edule (Linnaeus), the Common or Eatable Cockle, is known by the following characters :—It is neither triangular nor porcelain-white. It has radiating ribs, which are neither armed with spines nor tubercles. This bivalve assumes a variety of appearances, and the adult especially differs from the young. Forbes and Hanley include under this the following species of other writers :—C. vulgare, Da Costa; C. crenulatum, Lamarck; C. pectinaturn, Lamarck ; C. arcuaturn, Reeve; C. zonatum, Brown; C. obliquum, Woodward; C. rusticum, Chemnitz ; a glaucum, Brugiere ; C. Lamarckii, Reeve; a Belticum, Reeve.
This species is met with in most parts of the British Islands, and is almost everywhere regarded as a pleasant article of diet. The ordinary run of examples are from four-fifths of an inch to one inch iu length, but ou the coast of Devon, and especially at Limpstone on the mouth of the Ex, where they are cultivated in beds, they attain a much greater size. It is a gregarious animal, inhabiting the sands at low water, especially where there are large tracts of sand in the neighbourhood of mstuariea.
The Common Cockle has a wide geographical range, extending southward to the Canary Isles. It is also found in the Caspian Sea. It occurs fossil in the Red Crag.
The other British species enumerated by Forbes and Hanley, are— C. aculeatunt, C. C. rusticum, nodosunt, C. fasciatuni, C. pygnueum, C. Suecicum, and C. Norwegicurn. They regard C. Greenlan dicunt, C. serratum, a medium, and a muricatum, as spurious in th British' Fauna.