358 America}! Seashells 



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. 



