As facts relative to the earliest forms of Molluscs are of interest to the naturalist as well as the geologist, I offer this note on the genus Stenotheca, derived from a study of the remains of several species occurring in the Cambrian rocks of Canada. The formation in which they are found is the St.-John Group, a formation occurring in the Southern part of the Province of New Brunswick; and the geological horizons are those of the Solva and Menevian Groups of Wales, equivalent to the Paradoxides-Tesseni beds, and overlying measures of Étage 1, of the Norwegian geologists. The fossils are, therefore, of great antiquity, having preceded any known Lamellibranchs; and, except the Metoptoma Barrandei of Linnarsson, I know of no Gasteropods of equal antiquity.