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358 America}! Seashells
Superfamily PTERIACEA
Family ISOGNOMONIDAE
Genus Isognomon 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
South half of Florida and the West Indies.
Plate 35b
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.
hognomon radiatus Anton 1839 Lister's Tree Oyster
Southeast Florida and the West Indies.
plate 35a
% 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.