RHYNCHOTRETA

extinct genus of brachiopods (lamp shells) commonly found as fossils in Silurian marine rocks (between 438 and 408 million years old). Its small, roughly triangular shell is prominently ornamented by distinct ridges that run lengthwise to the shell margin. Because of its limited time range, Rhynchotreta is a good guide, or index, fossil for Silurian rocks and time.