Family Noetiidae

Shell size to 65 mm; shell heavy, stout, trigonal. Umbones well separated. Sculpture of about 32 strong ribs squarish in cross-section; finely incised lines present only on interspaces between ribs. Periostracum dark-brown or black. Shell white to dirty-white. The ponderous ark is one of the most common shells found on the beaches of Sanibel and Captiva islands. One of the most striking features of this species is its dark-brown or black, velvet-like periostracum, the organic shell layer that covers part of the shell or the entire shell. Visiting beachgoers often confuse the periostracum of the ponderous ark for a layer of oil or tar that could have deposited on the white shell. Another feature that sets the Ponderous Ark apart from other local species of mollusks is its association with the false sea fan, Leptogorgia hebes Verrill, 1869, a marine colonial organism related to sea whips, sea anemones, and corals. False sea fans attach themselves to the posterior region of Ponderous Ark shells (supplementary image), and may profit from an enhanced food supply (zooplankton) facilitated by the water currents created by the filter-feeding host bivalves.

Shell size to 17 mm; shell moderately inflated, elongate-oval to rectangular, depending on how sharp the "corners" are. Shell surface cancellate, sometimes with tiny beads at intersection of radial and commarginal ribs. Periostracum very thin or lacking. Shell color whitish, sometimes with cream or yellowish hues. Slightly damaged valves (see photos) found on Turner Beach, Captiva, by Susan J. Hewitt, in December 2016