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 



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- 



