Ordovician material from the Platteville Formation (Sandbian) of southern Wisconsin and Big Hill Formation (Katian) of northern Michigan is described that provides novel information about the phylogenetic affinity, taxonomic diversity, and stratigraphic range of the nonbiomineralized taxa Buthograptus, Callithamnopsis, and Chaetocladus. Two new species of Buthograptus, a previously monotypic genus, are erected on the basis of the Platteville Formation material, Buthograptus gundersoni n. sp. and B. meyeri n. sp., and new occurrences of B. laxus are recorded from several localities and two distinct stratigraphic levels within this unit. On the basis of scanning electron microscopic investigation of the material and the fact that each of the three Buthograptus species has a close counterpart in the frond morphology of an extant species of Caulerpa, Buthograptus is interpreted as a member of the green algal order Bryopsidales. New specimens from the Platteville Formation assigned to Callithamnopsis reveal new morphological details for the type species, C. fruticosa (Hall, 1865), aspects of which indicate that the genus belongs to the family Triploporellaceae rather than Seletonellaceae within the green algal order Dasycladales, and Chaetocladus material from the Big Hill Formation includes specimens that are formally assigned to Chaetocladus dubius (Spencer, 1884), a species of dasycladalean alga known previously only from the mid-Silurian of Ontario.