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The only species afterwards sent to me were Cerithia; but no vigilance sufficed to catch the desired sounds, and I still hesitate to accept the dictum of the fishermen, as the same mollusc abounds in all the other brackish estuaries on the coast; and it would be singular, if true, that the phenomenon of its uttering a musical note should be confined to a single spot in the lagoon of Batticaloa.
A similar geometry is found in the other shells with elongated cones, Turritellae, Spindle-shells, Cerithia, as well as in the shells with flattened cones, Trochidae, Turbines. The spherical shells, those whirled into a volute, are no exception to this rule. All, down to the common Snail-shell, are constructed according to logarithmic laws.
But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the Neritae, the inner margin of the outer lip toothed or crenulated. Potamides cinctus, Sowerby. The Potamides inhabit the mouths of rivers in warm latitudes, and are distinguishable from the marine Cerithia by their orbicular and multispiral opercula.
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.
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