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Occurrence of microdrills and modified shell at American Bottom sites.
Yerkes, R.W. 1991. Specialization in Shell Artifact Production at Cahokia. In New Perspectives on Cahokia, edited by James B. Stoltman, pp. 49-64. Monographs in World Archaeology No. 2 Prehistory Press, Madison. Table 3.2, p.59
Image courtesy of Prehistory Press.