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336 Genus Acila H, and A. Adams 1858 Similar to Nucula but characterized by the presence of divaricate sculp- ture on the outside of the shell. One common species in North American waters. Subgenus Trunc acila Schenck 193 1 Shell without the shallow sinus as seen in true Acila, and the posterior end of the shell nearly at right angles. Acila castrensis Hinds Divaricate Nut Clam Figure 72c Bering Sea to Lower California. H inch in length, abruptly truncate at the anterior end. Divaricate, radiating ribs plainly visible. Commonly dredged from 4 to 100 fathoms in sandy mud. Family NUCULANIDAE Genus Nuculana Link 1807 (Leda Schumacher 1817) Nuculana pernula Miiller Miiller's Nut Clam Arctic Ocean to Cape Cod. Northern Alaska. % to I inch in length, elongate and truncate posteriorly, moderately fat, slightly gaping at the rounded anterior end. Numerous raised, concen- tric growth lines. Periostracum light-brown to dark green-brown, semi- glossy. Shell dull-white, interior shiny-white. Interior of rostrum (posterior end of shell) reinforced by a strong radial roundish low rib. Lunule long, prominent, with sharp edge. Commonly dredged offshore in cold water. N. conceptionis Dall is much more elongate, smoother and glistening brown. Nuculana minuta Fabricius (Arctic to off San Diego; and to Nova Scotia) is /4 inch in length, % as high, rather plump, and with a short ros- trum whose smoothish lunule is bounded by a rather coarse rib. Concentric, raised threads are numerous and crowded. Beaks are one third to almost one half the way back from the rounded anterior end. Uncommon offshore. Nuculana tenuisulcata Couthouy Thin Nut Clam Figure 71a Arctic Seas to Cape Cod. Up to % inch in length, elongate, moderately compressed; rostrum mod- erately long with ^ sharp, high keel down the dorsal center (margin of the