Tivela tripla
Shell trigonal, with right and left valves of equal size and shape (equivalve); rather variable in elongation and inflation; maximum shell inflation coinciding with the diagonal ridge (carina); umbones central to slightly posterior, and only slightly turned towards the anterior end of the shell (nearly orthogirous); antero-dorsal, ventral and postero-dorsal margins slightly convex externally; lunule lanceolate, long, occupying two thirds of the antero-dorsal margin, flush with the general surface of the shell, delimited by a thin but well-defined ridge; escutcheon indistinct; posterior end of the shell flat in frontal view, set off from the remainder of the shell flank by the diagonal ridge (carina); shell exterior light yellow to brown, with or without commarginal and/or radial lines of purple pigment superimposed; external surface smooth, but commonly covered by blotches of a white, flaky, dehiscent coat (comprised of long, very fine calcified needles, only visible under a microscope); internal surface of the shell lacking pigmentation or with a tint of purple in the umbonal cavity, which may extend almost to the shell borders; internal shell margins without crenulation; pallial sinus rounded, deep, extending further anteriorly than the umbones, partially fused with the pallial line; ligament external, short, supported by a nymphal ridge in each valve; nymphal ridges rugose, with a tooth-like projection extending from their ventral edge, particularly conspicuous in the right valve; hinge of the right valve with two anterior lateral teeth (A1 and A3), three cardinals (3a, 1 and 3b) and a faint posterior lateral (P1); only the posteior cardinal (3b) is subdivided distally (bifid); hinge of the left valve with one anterior lateral tooth (A2), three cardinal teeth (2a, 2b and 4b), and a faint posterior lateral (P2); all teeth in the left valve entire (not bifid); lateral tooth A2 separate from anterior cardinal (2a); cardinals 2a and 2b dorsally united.
Original description, by Linnaeus (1771), as reprinted by Hanley (1855, p. 553):
"testa subtriangulari laevi antice posticeque retusa. Habitat... Testa laevissima, magnitudine extimi pollicis, retusa s. ani regione versus vulvae regionem compressa, ut fere triangula ; adeoque latus anterius et posterius respectu ad ipsam testam perpendicularia. Fornix et latus posterius intus violacea. Vulvae vix vestigium. Nymphae latentes, angulatae, solidae. Anus minutus. Margo integerrimus."