A statistical study of frequency distribution of number and pattern of lateral costae was made on three species of Anthracospirifer Lane, from samples collected from three separate beds in the lower part of the Bird Spring Formation (Lower Pennsylvanian) in Clark County, Nev. The study shows that Anthracospirifer birdspringensis has a significantly small total number of lateral costae and a large number of bifurcations per flank; A. opimus has a small number and few bifurcations of lateral costae; A. occiduus has a larger mean number of lateral costae and common bifurcations of the costae. Analyses of patterns of costae, as described, would probably serve to distinguish Anthracospirifer from Neospirifer Fredericks or other genera of Late Paleozoic spiriferids.

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