Page:American Seashells (1954).djvu/440

358 Superfamily PTERIACEA Family ISOGNOMONIDAE Genus Iso gnomon Solander 1786 Shell thin and greatly compressed; interior pearly; anterior margin with a narrow byssal gape near the dorsal margin. Hinge with numerous parallel grooves perpendicular to the dorsal margin of the valve. Perna Bruguiere and Pedalion Dillwyn are synonyms. Pedalion Solander 1770 is invalid. tsognomon alatus Gmelin Flat Tree Oyster Plate 35b South half of Florida and the West Indies. 2 to 3 inches in length. Hinge has 8 to 12 oblong grooves or sockets into which are set small, brown resiliums. Exterior with rough or smoothish growth lines. External color drab purplish gray to dirty-gray. Interior moderately pearly with stains of purplish brown or mottlings of blackish purple. This very flat, oval bivalve is commonly found in compact clumps on mangrove tree roots. Distinguished from /. radiatus by its flat, more regularly fan shape and darker color. ho gnomon radiatus Anton 1839 Lister's Tree Oyster plate 35a Southeast Florida and the West Indies. % to 2 inches in size, very irregular in shape, commonly elongate. Sometimes twisted and irregular. Hinge short, straight and with 4 to 8 very small, squarish sockets. Exterior rough with weak, flaky lamellations. Color a solid, translucent yellowish, but commonly with a few wavy, radial stripes of light purplish brown. Common on rocks at low tide. Formerly /. listen Hanley 1843. Isognomon bicolor C. B. Adams (Lower Florida Keys, Bermuda and Caribbean) is heavier, more oval, and commonly with strong lamellations on the outside. It is usually darkly and heavily splotched with purple inside and out. Common. According to Lamy, /. vulsella Lamarck is a different species which is limited to the Red Sea. The Western Tree Oyster, Isognomon chemiiitzianus Orbigny, from the Coronado Islands to Chili, lives in crowded colonies under stones in shallow water. It resembles the above two species, is about i to 2 inches in size; its right valve flattish, left valve slightly swollen. It is the only Cali- fornian Isognomon.