One hundred and forty-one species and subspecies of Late Cretaceous (Late Campanian) Foraminiferida are recorded in seven samples from a l05-meter section and the immediate vicinity of an exhumed, offshore oceanic ridge enveloped by claystone of the Rosario Formation near Valle El Morro, northwest Baja California, Mexico. Three faunules are recognized from the vicinity of the ridge. The richly fossiliferous Gavelinella multi-punctata Faunule is exhibited in strata lying on the ridge. The faunule is noted by a diverse assemblage of benthonic foraminifers and 35 per cent planktonic specimens, ostracods, nannoplankton, echinoids, brachiopods and mollusks, including Ostrea (?) and Spondylus, suggesting a warm, shelf environment supporting high organic production. Strata deposited adjacent to the ridge are characterized by the richly arenaceous Haplophragmoides spp. Faunule which possibly developed in an oxygen deficient environment beneath surface waters supporting high organic production. Calcareous elements of the faunule suggest increase in water depth upward through the claystone section, with upper bathyl depths at the top of the section suggested by abundant small, spinose buliminids of the Praebulimina joaguinensis Faunule.