Exogyra, extinct molluscan genus common in shallow-water marine deposits of the Jurassic and Cretaceous periods (from about 200 million to 65.5 million years ago). Exogyra is characterized by its very thick shell, which attained massive proportions. The left valve, or shell, is spirally twisted, whereas the right valve is flattish and much smaller. A distinctive longitudinal pattern of ribbing is well developed in the left valve, and pitting is common.