of the spire closely wound and not increasing much in diameter. A typical example is the decollate snail (Rumina decollata). The form of a shell The form of the shell of a gastropod is usually regular in coiling, and is normally a cone curved into a spiral, and descending in a screw-like manner from the apex or initial whorl to the aperture. The shell grows in a regular geometrical progression in its normal pattern, although these modes vary among themselves widely. Thus we have the simple depressed cone of Patella, all aperture and no spire. From it there is every gradation, from the Haliotis, almost equally depressed and broad, the result, however, of a very rapidly enlarging spiral, to the long, many-whorled Turritella or Vermetus, which is a Turritella partially unrolled into a simple long tube — the opposite of the Patella. Different types of spire See also : Gastropod shell#Shape of the shell Turbinoform or Turbinate: having a broadly conical spire and a convex base, as in Turbo, turban-shaped. Turriform: with a many-whorled, slender spire, as in Turritella. Chirality In most spiral shells the spire normally curves to the right, that is to say, placing the shell with its apex turned upward from the observer and its aperture in view : the aperture will be on the right hand side. In others the volutions proceed in the opposite direction with such regularity as to be eminently characteristic of some species and genera (Physa, Clausilia, etc.). However, in certain genera, it is found that species normally dextral will exceptionally produce sinistrally coiled shells, and vice versa. This abnormal growth probably is caused by disturbance of the relations of the embryo with its initial shell. References Mollusc shells Gastropod anatomy Cephalopod zootomy