(Philippi, 1841)
Description (shell):
Shell minute, transparent, with two and a half to three rapidly expanding, tumid whorls of almost circular cross-section. Sculpture of irregular growth lines and possibly low spiral ridges. Aperture with a thickened peristome; nearly circular in outline, slightly flattened or concave where it abuts penultimate whorl. Periphery of penultimate whorl lies slightly below medial diameter of aperture.
Size:
Up to 0.4 x 1.0 mm.
Colour:
Reddish-brown, cream, or yellow.
Animal:
Snout broad, bilobed, ciliate; no cephalic tentacles. Foot bilobed anteriorly; posterior mucus gland on sole. Operculum thin, horny, spiral, with a central nucleus (O. atomus-animal).
Habitat:
On algae, particularly Enteromorpha and Ulva ; common in pools below MTL, often suspended by mucus threads from surface; extends to 20 m.
Distribution:
Distributed from Azores to Norway; absent from Baltic and continental shores of North Sea, but recorded off Helgoland (Distr. O. atomus).