Spire (mollusc)

The space thus vacated is sometimes filled with solid shell, as in Magilus; or it is partitioned off, as in Vermetus, Euomphalus, Turritella, Triton or Caecum.

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.

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.

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.

Apertural view of the shell of adult Tarebia granifera showing its pale brown body whorl and dark spire.
Very high-spired shells of the sea snail species Turritella communis
Medium-spired shell (live individual) of a European land snail, probably Trochulus hispidus
Very low-spired shells of the land snail species Xerolenta obvia
The sinistral shell of the freshwater snail Planorbarius corneus , a view of the sunken spire, which is held facing downwards in life