Shell minute, with about three whorls lying in one plane so as to form a biconcave disk, glossy and rather transparent; nearly smooth, but with irregular, fine, scale-like rings marking growth lines. Animal lacks cephalic tentacles but has a pair of rounded, ciliated lobes, each with a central eye, formed from the snout. The whorls are more or less circular in section and are visible on both sides of the disk. Reddish brown. Up to 0.5 mm high, 1 mm across. The animals are hermaphrodite but the penis is not visible since it is retractile into a pouch.
Omalogyra atomus is found, often abundantly, on fine weeds growing on rocks and in rock pools on the lower half of the beach, and also to depths of about 20 m. The animals seem to eat the weeds and graze epiphytic diatoms. The species is widespread on British and Irish coasts except those of the southern North Sea. Its further distribution extends from the Mediterranean to Norway, Iceland. Greenland, and from Maine to Rhode Island.
These animals breed spring to autumn, laying more or less spherical egg capsules with a wrinkled surface about 200-300 µm in diameter, on Ulva and Enteromorpha. Usually there is only one egg in a capsule which emerges as a young snail in 2-3 weeks. Information on biology and breeding is given by Fretter (1948).