Shell brittle, obliquely oval, anterior hinge line sloping gently to convex anterior margin; posterior hinge line more steeply inclined, posterior margin only slightly convex. 18-23 ribs, each with a sharp central keel, regularly produced into erect, sharp spines, continuous basally with keel. On small specimens the spines are little more prominent than the keel, in later growth stages the keel is less obvious; old specimens may have both severely eroded; spines most prominent on anterior ribs. Concentric sculptures of numerous wavy ridges, pronounced between ribs, less so on them. Growth stages clear. Two cardinal teeth in each valve; right valve with two anterior and one posterior lateral teeth, left valve with single anterior and posterior laterals, the anterior being longer, thicker, and more prominent than posterior one. External sculpture visible on inner surfaces as grooves which extend the whole depth of shell. Adductor scars and pallial line indistinct (A. echinata-inside).
Up to 75 mm long.
Light fawn to deeper brown, in concentric bands, often darker or bluish-tinged along posterior margin. Periostracum thin, typically stained greyish by fine sediment. Inner surfaces white, glossy, occasionally bluish beneath umbones (A. echinata-drawing).
The animal varies in colour from white to pink. The gills are long and much folded back. The siphons are rather long. The flesh-coloured or red foot is large, long and finger shaped, twice the length of the shell. The mantle margins are jagged. Pigment spots serving as visual organs are scattered over the margins of the mantle and over the siphons.
In fine sand and gravel, usually with mud, offshore.
Common in the North Sea, ranges from Norway to the Mediterranean and the Canary Isles (Distr. A. echinata).