-Secretes calcium carbonate shell from mantle.
-Range in size from 1-100mm, although exceptionally can reach upto 500mm.
-The shell is usually coiled, typically helical.
-Classification of order is based on the shape and aperture of the shell.
-Reappear in Triassic sediments with very similar morphology to those which had seemingly disapeared; Lazarus taxa.
-As more fossils are found the similarities between pre and post P-T boundary faunas increases.
-May have lived in oceanic sediments, which have as yet not been found.
