Didacna pyramidata is a brackish-water bivalve mollusc of the family Cardiidae, the cockles. It has a high rounded-triangular, rather thick, cream shell, up to in length, with flattened ribs. The species is endemic to the Caspian Sea. It lives in the Southern Caspian sub-basin and in the southern part of the Middle Caspian sub-basin at depths between 30 and 100 m (98–328 ft), rarely down to .
Description.
"Didacna pyramidata" has a high rounded-triangular, rather 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.
Differences from 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, although its shells sometimes wash up on beaches. It differs from "D. pyramidata" by a more elongated and convex shell, with a more protruding umbo.
Distribution and ecology.
"Didacna pyramidata" is endemic to the Caspian Sea. It lives in the Southern Caspian sub-basin and in the southern part of the Middle Caspian sub-basin at depths between 30 and 100 m (98–328 ft), rarely down to . It does not occur in waters with salinity of less than 10–12‰.
Fossil record.
"Didacna pyramidata" occurs in the Holocene (Neocaspian) deposits of the middle and southern parts of the Caspian Sea. Nevesskaja (2007) hypothesized that it descended from the Late Pleistocene "D. praetrigonoides".
Taxonomy.
The species was first described as "Cardium pyramidatum" by Oscar Andreevich Grimm in 1877 from live individuals which were found in the southern part of the Caspian Sea off Azerbaijan at depths of . These specimens are now stored in the Zoological Institute of the Russian Academy of Sciences and the one collected at a depth of has been designated as the lectotype of "D. pyramidata" by Logvinenko and Starobogatov (1967). The coordinates of the type locality are 39°47'N 49°59'30"E.