What is "argyrotheca"

Argyrotheca is a genus of very small to minute lampshells (maximum long). All species share a large pedicel opening (or foramen), one ridge on the inside of the pedunculate valve, pits in a diamond pattern on the inside of both valves, and without radial ridges that end in tubercles. It occurs in depths between 6 and 1300 m. It is known since the latest Cretaceous.