Massive calcium carbonate mud mounds are present within the St. Joe Limestone (Lower Mississippian) of southwestern Missouri and northwestern Arkansas. These lenticles are up to 30 ft thick and 300 ft long. Thin-bedded crinoidal limestones enclose the bioherms. The enclosing limestones onlap the bioherms and by successive overlap extend over the mounds. Point-count analyses of thin sections from 9 localities were made. The bioherms consist mainly of calcite-mudstone (about 80%) with only a small percentage (approximately 13%) of skeletal remains and 5% sparry calcite. Fenestrate bryozoans are the most abundant skeletal constituents, making up about 8% of the rock. Echinoderm fragments comprise only 2% of the mounds. Other known skeletal components are present in a total amount generally less than 3%. The interbiohermal beds contain 47% skeletal grains including 13% fenestrate bryozoans and 25% echinoderm remains, 51% calcite mud and about 1% sparry calcite. The most acceptable hypotheses for the origin of the bioherms are: 1) the interaction of physical environmental factors and bathymetry; 2) the sediment-trapping and -stabilizing activities of fenestrate bryozoans, and 3) the sediment-trapping and -stabilizing activities of plants, not preserved. and 3) Although none of the 3 hypotheses can be completely rejected, it is possible that fenestrate bryozoans alone may have caused the growth and accumulation of these Missouri bioherms. It is also possible that the bryozoans were at least assisted by plants of which there are no preserved remains.