The attached fauna of one of the many hardgrounds from the Galena Group (Trentonian Substage) of the Upper Mississippi Valley is described. The fauna is composed of three principal elements, viz. (1) borers, including Cicatricula retiformis ichnogen. et ichnosp. nov., (2) pelmatozoans with encrusting holdfasts, and (3) bryozoans. Analysis of the distribution of members of each population on the hardground shows that most are strongly aggregated. The nature of, and reasons for, such aggregations are considered in the light of comparable Recent shallow‐water marine populations. The community on this hardground, and those on other Galena Group hardgrounds, are immature. This is a consequence of frequent and damaging scour, which these organisms were poorly adapted to resist.