Didacna pyramidata
Didacna pyramidata is a brackish-water bivalve mollusc of the family Cardiidae, the cockles. It has a high rounded-triangular, thick, cream shell, up to in length, with flattened ribs. The species is endemic to the Caspian Sea. It lives in the Southern Caspian and in the southern part of the Middle Caspian at depths between 30 and 100 m, rarely down to.
Description
Didacna pyramidata has a high rounded-triangular, thick, convex shell, with about 28–38 flattened radial ribs and a distinct posterior ridge which is sometimes marked by a stronger developed rib. The shell length is up to. The external coloration is cream, with thin brownish periostracum. The interior is whitish, with a yellow-brown stain on the posterior margin. The hinge consists of two cardinal teeth in the right valve and one cardinal tooth in the left valve. The right valve sometimes has weakly developed lateral teeth.Similar species
Didacna trigonoides has a more inflated shell, with a stronger protruding umbo, less ribs and a weaker posterior lateral tooth in the right valve.The fossil species Didacna subpyramidata has fewer ribs, a less distinct posterior ridge, higher umbo and its shell is usually more convex.
Didacna praetrigonoides is a possibly extinct species mostly known from the Late Pleistocene deposits of the Caspian Sea and the shells of which sometimes wash ashore. Compared to D. pyramidata it has a more elongated and convex shell, with a more protruding umbo.