Key to Bivalvia of the Deep-Sea
Hinge taxodont [c1ab], some deep water with subparallel teeth [c1c].
Hinge not taxodont or lacking teeth.
Ligament internal [c2] or as a small external arched band [c3ab].
Ligament external set on a flat [c4] or cleft [c5] dorsal area.
Shell thin, proteinaceous, greatly reduced anteriorly [c6], byssate, lacking true hinge teeth [c7], ligament external [c8]. Do not confuse dorsal row of crenulations with a taxodont hinge.
Shell typically calcareous, if reduced anteriorly then gaping or with true teeth, free-living, cemented or weakly byssate, ligament external or internal.
One valve cemented [c9] to or conforming to the substrate, if latter then lower valve with a foramen [c10] through which the byssus passes.
Lower valve with a byssus foramen [c10].
Ligament area curved [c11].
Ligament area symmetrical [c12].
Shells discoid to oval with projections (ears) [c13] extending from at least one side of the umbos.
Shells discoid, ligament internal [c14]. [Pectinoidea]
Shells obliquely oval to oval, ligament external in a triangular depression between the beaks [c15]. [Limoidea]
Boring into sunken wood within a fitting burrow. Shell globular with a large pedal gape, apophysis absent Xylophaginae [c16].
Not boring into rock or wood but may nestle in old burrows and crevices.
Dorsal margin of posterior area raised acute [c17], mesoplax not calcified.
Dorsal margin of posterior area broadly rounded [c18], mesoplax of 2 elongate triangular plates.
With at least 2 radiating cardinal teeth in LV [c19], ligament external [c20] or partly internal on a chondrophore [c21].
Hinge not with this combination of characters.
Pallial sinus present [c22], shallow to deep.
Pallial line entire [c23].
Subcircular or subovate shells, pallial sinus very deep [c22]. Both valves with 2 cardinals, ligament wholly external [c20] or partly internal on chondrophore [c21], cruciform muscle scars present. [Tellinoidea]
Trapezoidal-modioliform shells [c24] often distorted, teeth becoming eroded, posterior area with two rows of blunt spines.
Shell subcircular, ligament wholly external [c20], sculpture of fine raised commarginal threads [c25].
Shell subovate, ligament partly internal on a chondrophore [c21], sculpture smooth.
Posterior slightly more acute than anterior [c26], beaks low, radially striated (bathyal).
Posterior subacute, [c27] anterior broadly rounded, beaks projecting, posterior dorsal margin weakly serrated [c28] (abyssal).
Hinge with 2 cardinals [c19] and anterior and posterior laterals [c28], most with radial ribbing [c29]. [Cardioidea]
[Allen (2008) records the Arctic Clinocardium and Acanthocardia echinata from the deep Atlantic but we have no living records of either at bathyal depths. Any cardioid shells shoulf be identified using the main Cardioid Key.]
Hinge with 2-3 cardinal teeth only [c19]. [Astartidae]
Sculpture of commarginal lines only [c31], periostracum heavy brown to black (subarctic).
Sculpture of commarginal ridges [c32], periostracum yellowish to brown.
Sculpture of well spaced, rounded commarginal ridges [c32], anterior blunt.
Sculpture of crowded, narrow raised commarginal ridges [c33], anterior roundly pointed.
Ligament external or partly external, visible along the dorsal margin of joined valves [c34].
Ligament internal on a shallow resilifer or projecting chondrophore, not visible on the dorsal margin of joined valves.
Hinge glossoid, cardinal teeth subparallel with hinge line [c35].
Cardinal teeth reduced or absent.
Shell small less than 5mm, subspherical.
Shell elliptical to 20mm [c36].
Escutcheon weak [c37], lunule with faint demarcating line, posterior margin a continuous curve [c38].
Escutcheon long and excavated [c39], lunule strongly demarcated [c40], posterior margin with a distinct junction between escutcheon and post ventral region.
Thin, rhomboidal to polygonal shells [c41, c42], ligament external mostly sunken [c34], teeth absent or poorly formed.
Oblong to subovate shells, ligament partly internal on a shallow projecting resilifer [c43], teeth absent, many pustulose. [Thracioidea]
Truncated elongate-oval, beaks just behind midline, pallial sinus shallow, broad not reaching point under resilifer [c44].
Thruncated elongate -oval, beaks distinctly behind midline, pallial sinus narrow reaching point under resilifer [c45].
[These species are reported by Allen (2008) but we have no records of living species in bathyal or abyssal depths.]
Shell rhomboidal, weakly carinate [c46], ligament raised external [c47], teeth absent.
Thin, subovate to polygonal shells [c41, c42], ligament deeply sunken, teeth absent or rudimentary. Foot vermiform [c48] [Thyasiroidea]
Shells with a distinct posterior extension, rostrate [c49] or subrostrate [c50]. [Cuspidariidae]
Chondrophore projecting in both valves [c51], umbonal stress fracture present.
Chondrophore not projecting or not present.
[Allen (2008) lists Pandora pinna from the deep Atlantic, P. pinna would key out here but we have no records of this species living beyond the shelf edge.]
Shells nacreous with or without a shallow chondrophore, ligament supported by a lithodesma, teeth rudimentary, mostly deep water species with some granular microsculpture. [Lyonsiidae, Verticordiidae & Poromyidae]
Shells not nacreous ligament inserted on the hinge plate, or in the umbonal cavity or attached to a resilial shelf, mostly small shelf species, many commensal, shell lacking granular microsculpture. [Galeommatoidea ]