Fossil mollusks are common and conspicuous objects in shallow marine strata of Ordovician and younger ages, and so the broad features of the post-Cambrian history of the Mollusca have been known for many years. However, as all but one of the eight classes of the Mollusca had evolved by the beginning of the Ordovician, it is obvious that the pre-Ordovician history of the Mollusca holds the key to an understanding of the primary radiation of the phylum. It was with this view in mind, that John Pojeta, Jr. and I began our joint studies of Cambrian mollusks some 13 years ago.