Wikipedia
Isognomon
(For the tree oyster mushroom, see Pleurotus ostreatus.)
Isognomon is a genus of marine bivalve mollusks which is related to the pearl oysters.
Isognomon is known in the fossil record from the Permian period to the Quaternary period (age range: 254.0 to 0.012 millions years ago). Fossils of species within this genus have been found all over the world.