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360 Afjierican Seashells
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
Southeast Florida and the West Indies.
Plate 27W
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
North Carolina to south half of Florida and the Caribbean.
Plate 27X
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
North Carolina and south half of Florida.
Plate 27V
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. Commonly
washed ashore with rigida.
Superjamily PECTINACEA
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-