Genus CYCLOSTOMA.
A subdiscoidal or conic univalve. The aperture orbicular, with a circularly continued margin, often suddenly and widely reflected.
Species.
C. tricarinate. Shell with three volutions; three revolving, carinate, prominent lines, giving to the whorls a quadratic, instead of a cylindric appearance. Suture canaliculate, in consequence of the whorls revolving below the second carina and leaving an interval. Spire convex, apex obtuse. Umbilicus large. Carina placed, one on the upper edge of the whorl, one on the lower edge, and the third on the base beneath. Breadth one-fifth of an inch.
Inhabits the river Delaware. Rare.
Found by Mr. Le Sueuer, whose proposed name is here adopted.
C. lapidaria. Shell turreted, subumbilicate, with six volutions, which are obsoletely wrinkled across. Suture impressed. Aperture longitudinally ovate-orbicular, operculated, rather more than one-third of the length of the shell.
Length about one-fifth of an inch.
Collection of the Academy of Natural Sciences.
Inhabitant not so long as the shell, pale; head elongated into a rostrum as long as the tentacula, and emarginate at tip; tentacula two, filiform, acuminated at tip, short; eyes prominent, situated at the external or poste-
