The Ludlow Series of the Welsh Borderland represents a gradient from quiet. deep shelf muds to shallow, storm‐deposited silts. A fauna of 44 bivalve species shows a pattern of overlapping environmental ranges which span the full extent of the shelf gradient. Bivalve abundance is highest at the extremes of this gradient, where endobyssate forms are common in storm‐deposited silts, and nuculoids locally dominate the fauna of the deep shelf muds. Species diversity of bivalves, however, is highest in low‐stress. mid‐shelf environments, where bivalves of small population sizes comprise a minimal proportion of the benthic fauna. Niche sizes, species diversity. and total environmental range of the bivalves is comparable to that of Ludlow brachiopods, even though the latter group is far more abundant as indi