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360 buried, and there is a mass of byssal threads attached to small stones or frag- ments of shells. The broad end of the shell projects above the surface of the sand an inch or so. In the genus Pinna, there is a weak groove running down the middle of each valve. In Atrina, this character is absent. Pinna carnea Gmelin Amber Pen Shell Plate 27W Southeast Florida and the West Indies. 4 to 9 inches in length, relatively narrow, thin-shelled and with a central, radial ridge in the middle of the valve which is more conspicuous at the pointed or hinge end. With or without 10 radial rows of moderately large, scale-like spines. Color usually a light orangish to translucent-amber. Rare in Florida but common in the Bahamas. This is the only Pinna in the western Atlantic. P. nidis Linne is apparently a Mediterranean and West African species which is heavier and darker red. The name P. haiidigjjobilis Karsten is invalid, as are all this author's names. Genus Atrina Gray 1842 Atrina rigida Solander Stiff Pen Shell Plate 27X North Carolina to south half of Florida and the Caribbean. 5 to 9 inches in length, relatively wide, moderately thick-shelled and with 15 to 25 radial rows of tube-like spines; rarely smoothish. Color dark to light brown. Commonly washed ashore. A small, commensal crab lives inside the mantle cavity. A number of unusual snails and chitons are found in or on dead or live Pinna shells. Atrina serrata Sowerby Saw-toothed Pen Shell Plate 27V North Carolina and south half of Florida. Similar in size and shape to rigida, but covered with many more, much smaller, sharp spines. It is usually thinner-shelled and lighter in color. Com- monly washed ashore with rigida. Super jamily PECTIN ACE A Family PLICATULIDAE Genus Plicatula Lamarck 1801 Shell trigonal or spathate, thick-shelled and attached by either valve to rocks or other shells. Sculpture of broad, radial ribs. Hinge with a nar-