Natica. This genus of Lamarck's is characterized by an early globular shell, umbilicated at the left lip, with a callosity at the umbilicus ; the opening semicircu lar; the columella oblique, and not den tated.
Helicina. This genus is founded by Lamarck on a shell figured by Lister (Plate LXXI. fig. 59.) and which he de scribes as a globular shell ; the opening entire, semioval; columella with a callus, compressed inferiorly; and operculum. The propriety of this adoption cannot be judged of from merely the figure of this shell in Lister's work.
Helix. An orbicular or elongated uni valve, with an obtuse or concave spire ; the opening entire, forming a half moon. Lamarck divides the shells which have been hitherto disposed tinder this genus into eight genera. 1. Bulimus, an oval or oblong shell, the opening whole, long er than wide ; the columella smooth, with. out folds, truncature or widening, at the base ; as in Bulima hamastoma. 2. Lym nra, an oblong subturriculated shell, long er than wide; opening entire, the inferi or part of the right edge turning up, and passing into the opening, and forming a very oblique fold on the columella ; as in Helix stagnalis. 3. Melania, a turriculat ed shell; the opening entire, oval or oblong, widened at the base of the colu mella; Helix amarula is of this genus. 4. Ampullaria, a globular berried shell, umbilicated at its base, no callosities on the left lip ; the opening entire, longer than wide. Helix ampullacea forms this genus. 5. Planorbis. A discoid shell, the spire flattened or sunk, not promi nent, the opening entire, longer than wide, and filled up laterally by the con vex projection of the last turn but one. The type ofthis genus is Helix planorbis. 6. Haliotidea. 7. lanthina. 8. Helix, a globular shell, with a convex or conoidal spire ; and particularly with the opening diminished by the projection of the last turn but one ; but as this last character is common to the Planorbis also, I hese two genera are evidently confounded. The Separation, which is hereafter made of some shells, which originally were in the genus Helix, in agreement with the ideas of Brugulere, will be, therefore, more cor rect.
Volvaria, a cylindrical shell, twisted on itself, without a projecting spire ; the opening narrow, as long as the shell ; one or more folds on the base of the columel la. This genus was formed by Lamarck,
from a shell which is figured and describ ed by Pennant, vol. iv. Plate LXX fig. 85 He considers it as an intermediate between Bulla and Bulla. A tumid shell, the spire not pro jecting: the opening as long as the shell; no umbilicus. This genus of Linnzus has been much reduced by Brugulere, who placed several species under Billima, and established his genus Ovula. Lamarck has still further reduced them, by form ing the genera Terebellum, Pyruia, Am pulla, and Achatina.
Ianthina. This genus is formed by La marck on a single shell described by Lis ter, Brown, Forskal, and other natural ists; which derives its claims of distinc tion from Helix, not so much from the character of the shell as from that of the animal, which differs in its structure ma terially from the animal of the Helix, since it is furnished with a curious appa ratus, being an inhabitant of the sea, for swimming, instead of that for crawling, with which the Helices are provided. It is nearly globular, and almost diapha nous; the opening is nearly triangular, with an angular sinus at the right edge.
Turritella. This genus was formed by Lamarck for the reception of those shells which Linnzus had placed in the last sec tion of his genus Turbo, as Towers; and which Dargenville and Favanne had con sidered as screws. The characters of the genus are, a turriculated shell ; the open ing rotund, whole, but having a sinus' at the right edge.
Cyclostoma. This genus was formed by Lamarck for the reception of the Wentletrap, and other shells of the same character. The propriety of thus form ing' a new genus is rendered evident, by the doubts which had arisen respecting the placing of it in any old genus. Lin nzus having considered it a Turbo ; Rum phins, a Duce intim ; Dargenville, a Screw; and Davila, Gialteri, Guettard, and Fa wanne, merely a Tube, from its not pos sessing a columella, which they consider ed as essential in a univalve shell. The shells of this genus differ in their forms ; their openings are nearly round, Mid their sides connected circularly. The shell is formed of seven spires, separated by a void space ; with ten or twelve longitu dinal ribs, which, uniting, form a rim round the lip These ribs externally sup ply the place of a columella.