Adult snails in this genus have an extremely unusual shell morphology: the aperture of the adult shell faces directly “upwards”, in other words, in the same direction as the spire. This seemingly impossible arrangement is made possible because the adult shell is carried upside down.

In 1901, the American malacologist Henry Augustus Pilsbry commented that the adult shell of Anostoma is “so bizarre that in the total absence of information upon its life history, no useful theory can be formulated to account for its peculiarities.” This is one of the most peculiar genera of land snails.

The prominent feature of an upturned aperture (causing the adult snail to carry the shell spire down) is reflected in its scientific name Anostoma: ano, means up, or backwards, and stoma means mouth, from the Greek. In this genus, the adult snails carry the shell completely upside down, with the umbilicus uppermost, and the spire facing downwards.

To make this extraordinary feat possible, the mature aperture of the shell is aligned in the same plane as the rather low spire of the shell. It appears that in juveniles, the shell is carried with the widest diameter vertically aligned, in other words, with the keel of the shell pointing up.

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