The preservation of organic structures lining the Stromatactis cavities of the Silurian (Late Niagaran) reefs of northern Indiana, particularly along their smooth lower surfaces and forming their extremities, indicate that they represent a framework of mat-like bryozoan colonies belonging to the genus Fistulipora. These bryozoan colonies which trapped and held together the surrounding calcisiltite were subsequently dissolved to an appreciable extent by circulating fluids. Through solution and erosion, their upper surfaces expanded upwards into a complicated pattern of ramifications which nevertheless preserved the general shape of the bryozoan colonies but accentuated the contrast between their upper and lower surfaces. The cavities, which often join in labyrinthic networks, may contain internal sediments consisting of coarse skeletal debris brought in by the circulating solutions before and during the chemical precipitation of the drusy calcite mosaic which generally represents the final filling. The interpretation of the Stromatactis cavities in the reefs as partly dissolved framework-builder bryozoan colonies provides a major constructive agent which explains at the same time the origin of the calcisiltite cores and the peculiar shapes of their cavities.