MONOPLEURA

genus of extinct and unusual bivalves (clams) found as fossils in Cretaceous rocks (144 to 66.4 million years old) and representative of a group of aberrant clams known as the pachyodonts. The thick, triangular shell in Monopleura is capped by a much smaller dome-shaped shell. In some of the pachyodonts, there were open passageways through the shell that allowed for the passage of fluids. Monopleura and other pachyodonts were sedentary in habit; the animal apparently grew upright with the pointed end anchored in the substrate.