Upper Cretaceous Belemnitella species, as exemplified by a study of B. praecursor based on material from the northeast slope of the Dnieper-Donets basin, USSR, exhibit a complex evolutionary pattern which is attributed to the interaction of continuing, essentially irreversible orthogenetic trends and temporary, essentially reversible quantitative shifts resulting in a multitude of morphologic forms at each stage in the evolution of the stock. The taxonomic implications of such evolutionary patterns are discussed with special reference to the need for a broader concept of paleontologic species.

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