*Images in development
Radial ribs low, obscure but visible as internal marginal crenulations.
Radial ribs elevated, prominent.
Shell oval H>L, initially smooth develping 45-50 faint radial riblets.
Shell subovate L<H, ititially with about 30 obscure ribs these fading and persisting only at margins (Arctic).
Median ribs with erect spines or tubercles arising from the midline of the radial ribs, potentially large species. [Acanthocardia]
Median ribs smooth or with low relief cross bars or ventral facing fluted scales.
16-18 widely spaced angulate ribs bearing spathulate spines.
18-22 ribs, bearing either thorn like spines or blunt tubercles.
Ribs rather rounded bearing erect blunt tubercles.
Ribs subacute bearing thorn-like spines.
Shell rather thin, posterior spines long and much longer than median spines, commarginal sculpture weak.
Shell robust, all spines short of similar length, commarginal sculpture rugose between ribs.
Shells obliquely expanded posteriorly with a distinct keel.
Shells more or less equilateral, not oblique and lacking a distinct keel.
Robust shell, anterior and median ribs with widely spaced blunt nodules, posterior ribs with few spines lost in old shells.
Thin shells lacking nodules on median ribs (lagoons and brackish).
Posterior angle weak outline obliquely ovate, ribs low, occasionally with a few spines on dorsal area (Baltic).
Posterior angle strong posterior spines large in juveniles worn in adults (lagoonal).
32-36 narrow angulate ribs with a periostracal frill arising from the midline of each rib (Arctic).
Less than 30 ribs at least some with cross bars, fluted scales or spines.
Ribs rather low profile, with weak cross bars only, RV with a single posterior lateral.
Ribs well developed with fluted spines, crossbars or spines or a combination of these.
Robust shells, ligament projecting above dorsal margin when viewed for the side, generally uniform white to brownish.
Thin shells, ligament not projecting above dorsal margin when viewed for the side, variously patterned yellowish and brown, (lagoonal, brackish)
20-24 ribs, outline ovate (L>H) with closely spaced thin ventral facing fluted scales crossing entire width of rib (Arctic).
More than 24 ribs, with narrow fluted scales, cross bars or spines or a combination of these.
Outline ovate (L>H), 26-28 ribs, anterior and median with cross bars, posterior becoming elevated into spines.
Outline subcircular to oval (L≤H), Median ribs smooth or with rounded nodules or fluted spines.
Oval shell with smooth median ribs, anterior ribs with cross bars, posterior ribs with short spines, interspaces very narrow.
Subcircular shells, median ribs with spines or nodules.
Median ribs with rounded nodules.
Median ribs with narrow ventral facing fluted spines.