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Family Arcidae

SHELL heavy, regular, box-like, with strong epidermis: ligament external; hinge with a series of comb-like teeth; mantle open; foot large, bent, and deeply grooved; gills oblique, united posteriorly to a web; ocelli in mantle margin; siphons wanting. Warm seas.

Genus ARCA, Linn.

Shells equivalve or nearly so, oval, or rather four-sided, strongly ribbed or cancellated, ventricose, covered with heavy epidermis; hinge straight, with many small, transverse teeth; beaks nearer anterior end of shell, separated by a wide, flat lozenge-shaped ligamental area; foot pointed, with a heel; mantle supplied with marginal ocelli; gills long, narrow; hearts two; blood red in some species; byssal gland wanting or well developed. About one hundred and fifty species distributed in all warm seas, from tide mark to 250 fathoms.

The Bloody Clam (A. pexata, Say) is well known because of its abundance and size, and the fact that it has red blood, a rarity among mollusks. The shell is solid, obliquely oval, with thirty or more ribs radiating from the hinge over the knob-like beak. The deep grooves are delicately cancellated. The epidermis is thick, dark and bristly. Inside the margin of each valve is a border of alternate ribs and grooves. The hinge has a comb-like series of teeth.

The bloody clam has no place among "economic mollusks," I believe. But the shells are important items in the equipment of the littlest children who spend happy hours building hills and valleys on the white beach sand. The concave of that strong scalloped shell makes of it a capacious scoop, and the curve of its hump just fits the hand of the small architect. Height, 2 inches. Length, 2 1/2 to 3 inches. Rhode Island to Georgia.

The Ponderous Ark (A. ponderosa, Say) is exceptionally short and thick, in body and shell. The end view is symmetrically heart-shaped. The strong ribs are crossed on the marginal half by a few concentric lines, deeply impressed. The lips meet in a straight line. The beaks are distant, prominent and inclined to turn forward. The margins bear a wide border of dark, furry epidermis. Length, 2 1/2 inches. Fossil valves of this shell are picked up on New Jersey beaches.

Habitat.--Cape Cod to Texas, West Indies.

Noah's Ark (A. Nooe, Linn.) belongs to the group of ark shells which attach themselves by a byssus to rocks. For this reason they were called by Swainson, Byssoarca, to distinguish them from the free-swimming species. These byssoarks throw out a glandular secretion, comparable to the horse mussel's tough rope, and the silken cables of Pinna. The byssus of the ark, however, hardens into a horny cone, made of thin plates. This the ark can cast off with great suddenness, and as promptly secrete another.

A Noah's ark is at best an irregular box. The prominent umbones are separated by a wide dorsal depression above the straight hinge line. They are near the anterior end of the shell, which slopes downward like the prow of a dug-out. A widending groove extends from the beak of each valve to the abruptly truncated rear margin. The hinge has about fifty fine teeth. The ventral margin gapes midway to let the byssus out. Farther back the white mantle edges are dotted with ocelli--eye--spots--small brown elevations, each made up of many facets. On one specimen of Noah's ark Patten counted 235 of these compound eyes, of varying sizes, in the two mantle borders. With these well-developed sight organs may be mentioned the presence of red blood.

The mollusk scrambles about in rock crevices, grubbing in crannies and among rubbish, and in its tender years the shell often gets "warped for life." The ornamentation, too, gets badly worn. Most specimens cast ashore are smooth, especially about the beaks. A perfect shell is covered with strong ribs, marked with tigerish streaks of brown and yellow. Inside, the colour is plain lavender. Length, 3 to 5 inches.

Habitat.--Mediterranean Sea, North Carolina to West Indies.

A. Americana, Gray, has a larger, more oblong shell, with more ribs, flat and each impressed with a median line. These characters distinguish the two species, which occur together from North Carolina to the West Indies.

A. incongrua, Say, as broad as long, with wide ridges on the thin, unequal valves, is about two inches long and broad. The right valve is smaller, its ridges crinkled, especially behind the blunt umbones. North Carolina to West Indies.

A. transversa, Say, is rhomboidal in outline broadening at the posterior end. Its ribs are deeply cut, marked by fine scaly striae, and crossed by a few deep, concentric lines. Epidermis brown. Length, 1 1/2 to 2 inches.

Habitat.--New England to Florida Keys.

The Cross-lined Ark (A. reticulata, Gmel.) a very small species with cancellated surface, is found on the beaches of southern California. Specimens of other Mexican species sometimes stray northward into these waters. Such visitants are rare. A reticulata is found, also, from Cape Hatteras to the West Indies.