*Images in development
Cardioidea
Radial ribs low, obscure but visible as internal marginal crenulations.
Radial ribs elevated, prominent.
Shell subovate L<H, ititially with about 30 obscure ribs these fading and persisting only at margins (Arctic).
Serripes groenlandicus
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.
18-22 ribs, bearing either thorn like spines or blunt tubercles.
Ribs subacute bearing thorn-like spines.
Shell rather thin, posterior spines long and much longer than median spines, commarginal sculpture weak.
Acanthocardia aculeata
Shell robust, all spines short of similar length, commarginal sculpture rugose between ribs.
Acanthocardia echinata
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.
Parvicardium exiguum
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).
Parvicardium hauniense
32-36 narrow angulate ribs with a periostracal frill arising from the midline of each rib (Arctic).
Ciliatocardium ciliatum
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.
Cerastoderma edule
Thin shells, ligament not projecting above dorsal margin when viewed for the side, variously patterned yellowish and brown, (lagoonal, brackish)
Cerastoderma glaucum
20-24 ribs, outline ovate (L>H) with closely spaced thin ventral facing fluted scales crossing entire width of rib (Arctic).
Goethemia elegantula
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.
Parvicardium scabrum
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.
Parvicardium pinnulatum
Subcircular shells, median ribs with spines or nodules.
