The same approach in using sea-level curves to refine time-rock correlations between widely separated regions also has an important application in resolving the tangle of dual stratigraphic nomenclature that often develops in neighboring regions divided by political boundaries. A good example is the Lower Silurian of the Michigan Upper Peninsula and Manitoulin Island, Ontario. Four peaks in sea-level fluctuation are recorded in both areas by coeval pentamerid communities stratigraphically intermixed with coral–algal and ostracode–vermiform communities indicative of shallower water conditions. Despite many similarities between the rock units containing these fossil communities, most are identified by different formation names on opposite sides of the U.S.–Canadian border. This study details the eastward thinning of strata across the boundary area. The Cordell dolomite of Michigan is extended to Manitoulin Island, but not the underlying Schoolcraft dolomite, due to the pinching out of beds that define its upper and lower contacts. The Fossil Hill Formation of Manitoulin Island is accordingly reduced but retained even though it contains some of the other subformational units represented in the Schoolcraft. Facies relationships between the Michigan Hendricks dolomite and its correlative Mindemoya and St. Edmund Formations argue for their valid maintenance. The relationship between the Byron and Wingfield Formations is less clear. Probably the Dyer Bay Formation should be extended to Michigan in place of the Lime Island dolomite.