Planispiral

Planispiral is a condition in which a tubicolous shell is coiled in a single horizontal plane and the diameter increases away from the axis of coiling.

This term is usually used for describing gastropod shells.

Many cephalopods and tubeworms can also have a planispiral shell (Allaby and Allaby, 1999).

View of the spire side of the planispiral shell of the freshwater snail Anisus septemgyratus . This shell has seven and a half whorls