Most of the living Cerithia inhabit the sea near the mouths of rivers, where the waters are brackish; so that their abundance in the marine strata now under consideration is in harmony with the hypothesis that the Paris basin formed a gulf into which several rivers flowed. Calcarina rarispina, Desh. a. Natural size. b. Spirolina stenostoma, Desh. a. Natural size. b. Triloculina inflata, Desh. a.