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with strong, vertical, smooth ribs; remainder of shell smoothish with only irregular growth lines. Commonly found boring into soft coral blocks. Lithophaga antillanim Orbigny Giant Date Mussel Plate 28k Gulf of Mexico and the West Indies. 2 to 4 inches in length, elongate, cylindrical and colored a light yellow- ish brown on the outside and iridescent cream inside. Sides of valves marked with numerous, irregular, vertical riblets. Fairly common in soft rocks in moderately deep water. Subgenus Myoforceps P. Fischer 1886 Lithophaga aristata Dillwyn Scissor Date Mussel Plate 29) Southern Florida and the West Indies. La Jolla, California, to Peru. H to I inch in length. Characterized by the pointed tips at the posterior end being crossed like fingers. Color yellowish brown, but generally covered by a smooth, gray, calcareous encrustation. Moderately common in soft rock. Subgenus Dibenis Dall 1898 Lithophaga bisulcata Orbigny Mahogany Date Mussel Plate 28n North Carolina, the Gulf of Mexico, and the West Indies. I to 1/4 inches in length, elongate, cylindrical and coming to a point at the posterior end. A sharp, oblique, indented line divides each valve into two sections. Anterior half of valve smooth, mahogany-brown, but com- monly encrusted with porous, gray, calcium deposits. Posterior end more heavily encrusted with a gray, porous covering which projects beyond the edge of the shell. A fairly common rock-boring species. Lithophaga plujnula kelseyi Hertlein and Strong Kelsey's Date Mussel Plate i()i San Diego north to Mendocino County, California. I to 2 inches in length, similar to L. bisulcata, but the calcareous matter on the posterior end is strongly pitted and furrowed to look like a wet, ruffled feather. Typical plmnula Hanley ranges from Lower California to Peru. Both fairly common in rocks.