Preliminary data indicate that fenestrate bryozoa form the major faunal component in the core facies of the "crinoidal bioherms" of Osagian (Mississippian) age exposed in southern New Mexico. The core facies of these bioherms consists largely of aphanitic calcite and sparry calcite within a meshwork of intact and partially comminuted fenestrate bryozoans. Some of the bryozoans are in growth positions. Whether in growth position or as broken debris, the bryozoa are considered to be of significance in the genesis of these abrupt mound and ridge-shaped structures. The aphanitic calcite is interpreted as primary lime mud that was indigenous to the core facies. Much of this may be of algal origin. Algal frame-builders are not recognized. The core facies grades laterally with abruptness into a flank facies of coarse crinoidal debris, and thence into cherty calcilutites of the "interreef" facies. The core facies of many bioherms formed above the level of adjacent contemporaneous sedimentation, and some bioherms had several hundred feet of depositional relief. Most crinoidal material is considered to be indigenous to the flank or "interreef" facies. The core facies biotic assemblage is tentatively interpreted to not constitute a true reef assemblage, although apparently capable of growth into somewhat turbulent waters. Fenestrate bryozoans are probably of genetic significance in the Mississippian "crinoidal bioherms or reefs" in the subsurface of north-central Texas, and may be important in those of northeast Oklahoma. The New Mexico bioherms appear similar to the "reefs" of the Waulsortien facies of western Europe and to some of the English reef knolls, all of comparable Mississippian age. Good preservation of the primary faunal constituents, the structural simplicity, and the excellent exposures indicate that much needed data on paleoecological interpretation of Mississippian "reef" problems should result from detailed analysis of the bioherms of southern New Mexico.