Most living planktonic foraminifera thicken their tests in increasingly deeper habitats. Sphaeroidinella dehiscens (Parker and Jones), widely distributed in lower latitudes of the Atlantic and Pacific oceans, is shown to be an aberrant, terminal form of Globigerinoides sacculifer (Brady). The 'dehiscens' stage is achieved in a small percentage of forms by secretion of a thick calcite crust upon the test of G. sacculifer. Thickening reaches a maximum at mesopelagic and bathypelagic depths (300-2,000 m). G. sacculifer first occurs in early lower Miocene sediments, and 'S. dehiscens' in late middle Miocene. Possible evolutionary development of the cortex, and classification problems of polymorphic species, are discussed.