Miniaceoliva miniacea (Röding, 1798)?
~55 ft. Daytime, momentarily exposed before back into the sand.
Agat Bay, Guam.
21 Sept 2008.
Note: The Miniaceoliva miniacea-efasciata-lamberti species complex is confusing, One can argue that Guam specimens are berti now considered a subspecies of efasciata, generally associated Marshall and Caroline Islands.