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Rh the valves. No scientific study has been made of this variant, but it may be due to environmental or dietetic conditions.

Figure 94. Mud, peat and rock borers of the Atlantic Coast, a, Angel Wing, Barnea costata Linne; b, Petricola pholadifonnis Lamarck, False Angel Wing (p. 420); c, Northern Piddock, Zirfaea crispata Linne. All slightly reduced. (From Gould and Binney 1870.) Barnea truncata Say Fallen Angel Wing Massachusetts Bay to south Florida. 2 to 2/4 inches in length, somewhat resembling the Angel Wing, B. costata, but widely gaping at both ends, truncate at the posterior end, smoothish at the posterior half, and with an elongate, narrow accessory plate over the beak area. Shell fragile, white to grayish white. The internal condyle is long and about the thickness of a toothpick. The siphons are encased in a large, rough, gray, tubular sheath which may be extended 2 to 3 times the length of the shell. Bores into clay, soft rock or wood. Common in intertidal zones. Do not confuse with Zirfaea crispata which is much more squat and has a radial, indented line dividing each valve into 2 areas. Barnea siibtnmcata Sowcrby 1834 Pacific Mud Piddock San Francisco, California, to Lower California.