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Family Mactridae

SHELL equivalve, three-cornered, heavy; hinge formed by two cardinal teeth with laterals; epidermis thick; siphon tubes united, fringed at tip; mantle open in front; foot flattened.

Genus MACTRA, Linn.

Shell thick, almost equilateral, anterior hinge tooth V-shaped. Widely distributed chiefly in tropical seas. One hundred and fifty species.

The surf clams live buried just below the surface of the sand. They extend the finger-like foot to a considerable distance in search of food; also use it in leaping. Star fishes and whelks are their enemies. They are eaten by man, and collected to feed pigs and to fertilise the soil.

The Solid Surf Clam (M. Solidissima, Chemn.) is the common "hen clam" of the New England coast. It burrows in sand, from which it is dug at low water with shovels or clam rakes. Reeve says that at high water fishermen go out, poking the bottom with sharp sticks. If a stick by chance goes between the open valves of a clam, it closes at once, and the fisherman draws it up.

The shell is indeed solidissima. Lines of growth sculpture the surface. The posterior end is long drawn out. The hinge leans forward. The umbones are prominent. When open, the valves show at the hinge the very large heart-shaped cartilage pit, with long channels in both directions, and blade-like teeth with cross striations. The muscle scars are oval and very large.

This is the largest bivalve on our Atlantic coast. It is ponderous, too, but so muscular as to be able to leap, by means of its extensible foot, when escaping from enemies. Among these are counted men, pigs, star fishes and whelks. Length, 7 inches.

The Beaked Surf Clam (M. nasuta, Gld.) is regularly elliptical, with a somewhat elevated beak on each side of the central hinge, and thin white valves. Length, 3 to 4 inches.

Habitat.--California.

The Californian Surf Clam (M. Californica, Conr.) looks like a little Mya, being thin-shelled, and flattened behind the central, furrowed beaks. Over the white surface is a yellow epidermis. Beaks very small, close. Length, 2 to 3 inches.

The Mattock Surf Clam (M. dolabriformis, Conr.) has the form of an axe head, and is thin, flat, and white. Length, 3 inches.

Habitat.--San Diego southward.

M. lateralis, Say, is triangular and swollen, with a wavy and bluntly ridged posterior surface, and a ridge to balance it near the front. The prominent beaks are nearly central. Length, 1 inch.

Habitat.--Atlantic coast.

M. fragilis, Chemn., is a delicate shell recognisable by the darker colour of the epidermis covering the posterior area, which is set off by distinct ridges. Elsewhere the epidermis is yellowish. Faint rays extend from the central compressed umbones to the wrinkled margins opposite. Lining white, polished. Length, 3 to 4 inches.

Habitat.--North Carolina southward.

Genus LABIOSA, Schmidt

Shell oblong, thin, widely gaping and reflected posteriorly; hind margin defined by a thick lip or keel.

Sub-genus Raeta is a small group of surf clams with widely gaping, thick-lipped valves. A sharp ridge defines the posterior area. The valves are almost heart-shaped, thin, with concentric folds on the surface.

The Channeled Raeta (L. canaliculata, Say) has the characteristics above--it is the type. The grooves cut deep, making a very fragile shell; though much swollen in the front half, it becomes suddenly flattened behind. Interior grooved. Length, 2 to 3 inches; width, somewhat less.

Habitat.--New Jersey southward.

L. lineata, Say, has a sharply keeled line setting off the anterior end, which gapes in a narrow line for the thin foot. The posterior gape is wider. The shell is white and very thin, closely marked with concentric lines. Length, 2 to 3 inches.

Habitat.--New Jersey southward.

Genus TRESUS, Gray

The Washington Clam (T. Nuttallii, Gray, T. maximus Midd., Scbizothaerus Nuttalli, Conr.) would doubtless, "by any other name" be still the giant of all the surf clams. Like many another of our molluscan giants, it is found on the Pacific coast. Tryon figures this shell as oblong, rounded at both ends. It is remarkable for the wide gape of its white valves at the posterior end, and the narrow gape in front. The body of the shell is much swollen, to accommodate the very large body. The blunt umbones come near meeting over the hinge, which has a deep, wide cartilage pit, and a long channel on each side of the cardinal teeth. A brownish epidermis covers the surface. This mollusk burrows deep in muddy bays. The laborious digger may need a helper to hold on to the clam's "neck" to insure its capture. He finds consolation in the fact that a large chowder may be made of a small number of clams. Dr. R. E. C. Stearns characterises this species as "a noble and estimable clam, which beats any other clam yet discovered for chowder, soup or pies. It has nearly the consistency of an oyster, a very small foot; the proportion of tough muscle is less than in Mya arenaria." Length, 6 inches. Puget Sound to Southern California.