Nuculopsis, extinct genus of clams found as fossils in rocks of the Pennsylvanian Subperiod (318 million to 299 million years ago). Nuculopsis was small, almost spherical, and ornamented with fine growth lines. Because Nuculopsis is similar to the longer lived and commoner genus Nuculana, it has been considered a subgenus of Nuculana. Nuculopsis inhabited the broad, shallow Pennsylvanian seas that covered much of midcontinental North America.