A general survey of the calcareous algae of the Silurian of the Welsh Border has been undertaken. The genera discovered during this investigation, and. so under consideration, are those of Girvanella, Rothpletzella, Solenopora and Rhabdoporella. Systematic descriptions are given for each, and their variation shown. Girvanella has been subdivided here into form-species, mostly new types, on the basis of character differences, combined with chronological, geographical and ecological fgotors. Both species of Rothpletzella together with a fine-tubed form and three species of Solenopora qre recorded. Only one species of Solenopora Ras originally known from this area and a second just outside. Rhabdoporella has been obtained from the Wenlockian, but was only previously recorded from the Llandovery of Meifod. The lateral and vertical distribution of each alga has been studied and as far as possible quantitative methods have also been used. These investigations have been in relation to the limestone facies and associated fauna. It is shown that algae, especially the filamentous forms, are much more abundant and are far more widespread than has been previously recorded. In some instances, their distribution appears to change less according to horizon level than to differences of limestone facies but others still appear to be of stratigraphical value. The abundant distribution of Solenopora and Rothpletzella species is mostly correlated with reef or reef-like conditions which existed during most of the limestone formation periods, but the distribution of the other genera is not thus correlated. The apparent preferences for different types at conditions by the various algae are indicated, and comparisons are made with the ecology of present day reefs.