bubble-shaped Bulla Coeloconoid a slightly concave conical shell in which the incremental angle increases steadily during growth (see: Calliostoma) Cone-shaped, obconic. Conus Contabulate, short, with shouldered whorls Convolute: aperture as long as the shell, nearly or quite concealing the spire. Cypraea Cylindrical, pupiform. Lioplax, Pupa Cyrtoconoid: approaching a conical shape but with convex sides (see: Gibbula) Depressed, lenticular. Ethalia carneolata Discoidal. Elachorbis Ear-shaped. Haliotis Elongated, subulate, elevated. Terebra Few-whorled. Helix pomatia. Fusiform, spindle-shaped. Fusinus Gibbous. Whorls swelled beyond the normal contour of increase (usually on the aperture side ). Streptaxis. Globular. Natica Many-whorled. Millerelix peregrina. Short, bucciniform. Buccinum Trochiform, pyramidal, conical with a flat base. Trochus Turbinated: conical, with rounded base. Turbo Turreted, turriculate, babylonic; an elongated shell with the whorls angulated or shouldered on their upper part. Turritella Scalariform, whorls not impinging. Epitonium scalare Irregularly spiral, evolute. Siliquaria, Vermetus Tubular. Shield-shaped. Umbraculum Boat-shaped, slipper-shaped. Crepidula Conical or limpet-shaped. Patella Biconic: shaped like two conical shapes that are touching their bases, and tapering at both ends: Fasciolaria tulipa Pear-shaped: a combination of two shapes: ovate-conic and conic. Ficus Detailed distinction of the shape can be: Dimensions The most frequently used measurements of a gastropod shell are: the height of the shell, the width of the shell, the height of the aperture and the width of the aperture. The number of whorls is also often used. In this context, the height (or the length) of a shell is its maximum measurement along the central axis. The width (or breadth, or diameter) is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal. The largest height of any shell is found in the marine snail species Syrinx aruanus, which can be up to 91 cm. The central axis is an imaginary axis along the length of a shell, around which, in a coiled shell, the whorls spiral. The central axis passes through the columella, the central pillar of the shell. Evolutionary changes Among proposed roles invoked for the variability of shells during evolution include mechanical stability, defense against predators and climatic selection. The shells of some gastropods have been reduced or partly reduced during their evolution. This reduction can be seen in all slugs, in semi-slugs and in various other marine and non-marine gastropods. Sometimes the