The numerous reefs outcropping in the region around Formosa, Ontario, have been previously considered by various authors as equivalent to the Alpena limestone of Michigan, the Delaware limestone of Ohio, the Onondaga limestone of New York, and the Detroit River group of southwestern Ontario, southeastern Michigan, and northwestern Ohio. Outcrop and subsurface lithostratigraphic evidence indicates that the last correlation is correct. By means of detailed taxonomic study of the Formosa reef fauna an attempt was made to confirm this evidence paleontologically. One hundred twenty-eight species and subspecies have been identified and illustrated of which 31 species and 2 subspecies are new, 35 have been previously described, and 59 cannot be positively identified because of poor preservation. Sixty-six percent of the previously described species and subspecies are of Onondagan age (=Bois Blanc, Detroit River, and Columbus), 16% occur in both the Onondagan and Hamilton, and 16% were heretofore known only from the Hamilton. Of the Onondagan species only 4 are known to be restricted to the Detroit River group; all 4 occur in the Amherstburg dolomite which suggests a correlation of the Formosa reef limestone with the Amherstburg.