The Mactridæ have an internal ligament to the hinge, the siphons are joined together and fringed at the ends, and the pallial line is more or less sinuated. Spisula solidissima (Case 184), the largest species found on the coast of the United States, is a common article of diet.
Fig. 32.
Common British Cockle
(Cardium edule).
a, foot; b, exhalant siphon; c, branchial or inhalant siphon; d, edge of mantle; e, ligament; f, umbones or beaks of the shell.
The next family, Veneridæ, have long respiratory siphons and a sinuated pallial line. Many of this tribe are very beautiful in form and colouring, and most of them have very hard strong shells. The valves are united above by an external ligament, and the hinge-plate is toothed. Nearly all of them live buried an inch or two beneath the sand or mud, but a few are found burrowing in rocks. Probably the majority of the species of this family might be used as food. Venus verrucosa, of our own southern shores, is frequently eaten both in this country and abroad; and Venus mercenaria (Case 189) is commonly sold in the markets of Philadelphia and New York. Cytherea lusoria (Case 185) also forms a favourite article of diet among the poorer classes in Japan, and several kinds are eaten by the natives of New Zealand and other countries.
Cases 192–194.
Some of the “Cockles” (Cardiidæ) from warm latitudes are highly coloured and adorned with most beautiful sculpture. Probably the majority are eatable, as the common cockle (Cardium edule) of the British coast. The foot of these molluscs is very large, bent, and used for leaping. The siphons are short and fringed at the margins.
Cases 195–196.
The Tridacnidæ, or true Clams, differ from other Bivalves with united mantle-margins in having but a central adductor muscle. In the typical species the animal is attached to the rocks by a “byssus,” a strong fibrous structure which passes through an aperture at the upper part of the shell. A species found in the Red Sea, T. elongata, is eaten by the natives, and the shell employed for the manufacture of lime. Tridacna gigas, the largest known bivalved mollusc, sometimes weighs over 500 lb., that exhibited in the upright cases at the entrance of the Gallery being 310 lb. in weight. A large pair bordered with gilt copper are used as bénitiers or holy-water vessels in the church of St. Sulpice in Paris. Tridacnæ are found associated in large numbers in lagoons, among coral-reefs in the Eastern and Pacific Seas. The animals are described as presenting a beautiful iridescent glare of blue, violet, and yellow variegated with fantastic markings.
Fig. 33.
Left valve of the Giant Clam (Tridacna gigas).
Length, 36 inches. Weight, 154 lb.; weight of the two valves, 310 lb.