Page:American Seashells (1954).djvu/563

Rh % to I ^ inches in length, elongate, moderately fragile, gaping at the posterior end and whitish in color. There is a faint radial groove from beak to ventral margin in each valve. Posterior to this groove, the shell is weakly sculptured with concentric growth lines. Anterior to the groove, there are crowded, finely denticulated riblets. Dorsal margins of valve joined by a long, narrow, fragile plate. Over the umbones there is a broadly rounded, somewhat heart-shaped, thick, calcareous plate. Anterior gape closed over by an egg-shell callus. Commonly found boring in wood. Martesia cuneijormis Say Wedge-shaped Martesia North Carolina to Florida, Texas and the West Indies. % to % inch in length, similar to striata, but smaller, chubbier, and with a somewhat triangular or lanceolate accessory plate which commonly has a crease running down the center. There is no free flange extending over the beak. Very commonly found burrowing in wood. Martesia smithi Tryon Smith's Martesia New York to north half of Florida and to Texas. % to ^ inch in length. Bores into rocks and old shells. Very chubby, with a narrow, pointed posterior end. The shelly plate over the beaks is somewhat diamond-shaped, with a pointed anterior end and with no crease on the upper surface. Posterior half of the valves with fine, silken, concentric threads of periostracum. Fairly common in old oyster shells. Family TEREDINIDAE Genus Bankia Gray 1842 This is a genus of shipworms which is very common in warm American waters and which is of great economic importance. The two highly spe- cialized valves of the clam are very similar to those in Teredo, and cannot be used as reliable identification characters. The two plume-like pallets at the posterior end of the worm-like animal which can close off the end of the burrow are used to distinguish species. In adult specimens, the shell at the anterior end which scrapes away the moist wood in the tunnel is about /4o the total length of the animal. The mantel secretes a thin, smooth cal- careous lining for the burrow as an added protection to the soft body. A much fuller and technical account of this group is given by Clench and Turner in Johnsonia (1946). The life history is explained in detail by Sigerfoos (1908). We have included all three known species in eastern American waters, although five others are found in the West Indies.