Nautilus crista Linnaeus, 1758 Armiger crista (Linnaeus, 1758) Planorbis crista (Linnaeus, 1758) Gyraulus crista, commonly called the Nautilus ramshorn, is a minute species of freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails.

The distribution type is circumpolar wide-temperate, and the range includes: The minute shell is nearly planispiral in its coiling, but the spire is actually considerably sunken rather than raised (which is usually the case with gastropods).

This snail lives on water plants in clear still freshwater.

In Ireland it is common in medium to very small habitats including slow streams, drains and marsh and fen pools It is occasionally found in very acid waters.

This minute species, although technically a pulmonate gastropod, does not use air for respiration but instead absorbs oxygen through its mantle cavity which is full of water.