Forty species of gastropods (six of them new: Calliomphalus (Planolateralus) mexcalensis, Nododelphinula diminuta, Potamides temalacensis, Ptychopotamides ancestralus, Amaurellina guerrera, and Drilluta gloriae), and 13 species of bivalves are described from the lower Maastrichtian marls of the basal Mexcala Formation from Guerrero State, southern Mexico. Age interpretation was based on planktic foraminiferans. Associated fauna includes planktic and benthic foraminiferans, ostracods, solitary corals, crustaceans, echinoids, and vertebrates. The distribution of bivalve and gastropod species suggests that this area has affinities with the Gulf Coast Plain Province of the United States, although some species also are reported from the Middle East, Europe, and South America.

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