The Rock-Dwellers Family Petricolidae

THE ROCK-DWELLERS FAMILY PETRICOLIDAE. Shell oval, thin, white, gaping behind; hinge with two or three teeth, no laterals; pallial sinus deep; epidermis thin; mantle thickened and recurved over shell at edge, closed in front, except for small pedal opening, foot narrow, pointed; siphons long, sep arate at least part way, tips fringed. Animal free, but frequently boring into clay and soft rocks.

Genus PETRICOLA, Lam.

Characters of the family.

The (P. Pholadiformis, Lam.) is a narrow and oblique oval, with corner beaks, rayed faintly all over, wrinkled into ridges on the short anterior area. The long siphons are divergent for almost their entire length. The species is common on sandy or muddy beaches of New Jersey. Farther south it finds a suitable hiding place in masses of coral sediment. Length, 2/ inches.

Habitat.— Atlantic coast.

P. carditoides,

Conr., adapts its form to the burrow it in habits. It often becomes very thick shelled and rough, and always dingy white. Preempting a suitable cranny when young, the shell grows to fit it. Length, I to 2 inches.

Habitat.— West coast.

Genus SAXIDOMUS, Conr.

Shell large, oval, thick, with three strong, divergent cardinal teeth in the hinge. Ligament large, external. Large edible clams of the northwest coast.

The Giant (S. giganteus, Desh.) has the gen eral form of the eastern hard-shell clam, but is larger, on an average. The extreme is between five and six inches in length.

345 The Rock-dwellers This is the best edible clam at Vancouver. It is the "Oregon clam" of the Portland markets. The shell is chalky, with upturned, concentric ridges, dingy, or yellowish white. The lining is always white and china-like.

Habitat.— Aleutian Islands to Monterey Bay.

S.

Nuttallii, Conr., is roughened by concentric ridges, and is coarse and dingy outside. Young ones have brown markings on the beaks, and purple stains inside the shell. Adults have a peculiar, clear, agate-like shell structure bordering the large, rubbery ligament. Length, 4 to 5 inches.

Habitat.— San Francisco to San Diego, Cal.

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