Ammonicera plana, a new species of Omalogyridae, is described from the infratidal zone of the north coast of São Paulo State, Brazil. Distinguishing shell characters are; five dark spiral bands per whorl; great lateral compression; poor development of shell sculpture and the presence of a lateral spiral ridge. The anatomy is compared with that of the European Omalogyra atomus, from which it differs; in having mixed testis and ovarium; no large glands near the anus and in lacking vesicula seminalis. The anatomical data support the inclusion of the Omalogyridae in the Allogastropoda rather than in the euthyneuran Rissoelloidea.
Amimonicera plana n. sp.