*Images in development
Major Taxa
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 [c3]
Protobranchia
Ligament external set on a flat [c4] or cleft [c5] dorsal area
Shell thin, proteinaceous, greatly reduced anteriorly [c10,c11,c12}, byssate, lacking true hinge teeth, ligament external
Pterioida & Mytiloida
Shell typically calcareous, if reduced anteriorly then gaping or with true teeth, free-living, cemented or weakly byssate, ligament external or internal
Shell not cemented or with lower valve with a byssus foramen
Shells discoid [c14] to oval [c15] with projections (ears) [c17] extending from at least one side of the umbos
Shells obliquely oval to oval, ligament external in a triangular depression between the beaks
Limoidea
Not boring into rock or wood but may nestle in old burrows and crevices
With at least 2 radiating cardinal teeth in each valve, ligament external or partly internal on a chondrophore or wholly internal
Hinge not with this combination of characters
Pallial sinus present, in some represented only by a slightly indentation of the posterior ventral line if so lateral tooth present
Hinge with 2 cardinals and anterior and posterior laterals, most with radial ribbing
Cardioidea
Adductor scars of similar shape and size
Both valves with 2 cardinals, laterals absent or present, some with partly internal ligament on chondrophore, cruciform muscle scars present, pallial sinus extensive
Tellinoidea
Ligament mostly internal in a shallow but wide resilifer, most have LV with inverted v-shaped cardinal teeth
Mactroidea
Ligament external or partly external, visible along the dorsal margin of joined valves
Ligament internal on a shallow resilifer or projecting chondrophore
Elongate, cylindical shells, LV with 2 cardinals RV with 1
Solenoidea
Subovate, polygonal or oblong shells
Cardinal teeth reduced or absent
Thin, subovate to polygonal shells, ligament external but sunken, teeth absent or poorly formed
Thyasiroidea
Oblong, subovate to rhomboidal shells
Oblong to subovate shells, ligament partly internal on a shallow projecting resilifer, teeth absent, many pustulose
Anomalodesmata
Shells with a distinct posterior extension, rostrate or subrostrate
Cuspidariidae
Hinge with a projecting, tooth-like, chondrophore in one valve, Corbiculidae very inequivalve
Myoidea
Chondrophore not projecting or if projecting then equally in both valves
Chondrophore projecting in both valves, umbonal stress fracture present
Anomalodesmata
Chondrophore not projecting or not present
Shells not markedly inequivalve
Shells nacreous with or without a shallow chondrophore, ligament supported by a lithodesma, teeth rudimentary, mostly deep water species with some granular microsculpture.
Anomalodesmata
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/Neoleptonidae
