Umbonium

Umbonium, sometimes known as the "button top shells", is a genus of sea snails, marine gastropod molluscs in the family Trochidae, the top snails.

==Shell description== The shells of the species in this genus are low-spired and shaped like a button. The orbicular shell is depressed and imperforated. It is polished, porcellaneous and has a very thin pearly layer inside. The whorls are flattened above, bright, smooth or spirally grooved. The small, transverse aperture is wider than high. The thin outer lip is acute. The inner lip is rounded, ending in a simple point. The umbilicus of the shell is often completely covered with a thick and smooth callus.