What is "neithea"

Neithea is an extinct genus of bivalve mollusks that lived from the Early Jurassic to the early Paleocene, with a worldwide distribution. Neithia sp. are inequivalve. That means that the two valves are not the same shape, the right valve being strongly concave and the left valve being flattened or concave. Sculpture consist of alternating strong and weaker radiating ribs.