Umbonium
Button top shells: low-spired, polished marine snails in Trochidae.
**Umbonium**, commonly called button top shells, is a genus of marine gastropod molluscs belonging to the family Trochidae, which includes the top snails.
The shells in this genus are low-spired and button-shaped. They are depressed, imperforate, and polished, with a porcellaneous surface and a very thin nacreous layer inside. The whorls are flattened on top, either smooth or marked with spiral grooves, and are glossy. The aperture is small, transverse, and wider than it is tall. The outer lip is thin and sharp, while the inner lip is rounded and ends in a simple point. The umbilicus is often entirely concealed by a thick, smooth callus.
The animal has a distinct lateral fringe on the foot, with three filaments on each side. On the right side, near the base of the tentacles, a fleshy lobe projects forward. The right tentacle is free, with a compressed eye-peduncle that bears a rudimentary eye. The left eye-peduncle is cylindrical, has a well-developed eye, and is equipped with a folded, fringed frontal lobe. The operculum is horny, round, and multispiral.
Species in the genus include *Umbonium callosum*, *U. conicum*, *U. costatum*, *U. elegans*, *U. eloiseae*, *U. giganteum*, *U. moniliferum*, *U. sagittatum*, *U. suturale*, *U. thomasi*, and *U. vestiarium*. Several names once placed in *Umbonium* are now considered synonyms, such as *U. adamsi* (synonym of *U. thomasi*), *U. bairdii* (now *Margarites bairdii*), *U. capillata* (now *Ethaliella capillata*), *U. depressum* (synonym of *U. vestiarium*), *U. floccata* (now *Ethaliella floccata*), *U. guamense* and *U. guamensis* (both now *Ethalia guamensis*), *U. javanicum* (synonym of *U. moniliferum*), *U. nitida* (now *Ethalia nitida*), *U. omphalotropis* (now *Ethalia omphalotropis*), *U. polita* (now *Ethalia polita*), *U. pulchella* (now *Ethaliella pulchella*), *U. rufula* (now *Ethalia rufula*), *U. sanguinea* (now *Ethalia sanguinea*), *U. striolatum* (now *Ethalia striolata*), and *U. zelandica* (now *Zethalia zelandica*).
Quick Facts
- Taxon
- Umbonium
- Type Species
- Trochus vestiarius
- Type Species Authority
- Linnaeus, 1758
Facts from the source article.
Lore & Background
The shells of Umbonium species are low-spired and shaped like a button. The orbicular shell is depressed and imperforated, polished and porcellaneous, with a very thin pearly layer inside. The whorls are flattened above, bright, smooth or spirally grooved. The small, transverse aperture is wider than high, with a thin acute outer lip and a rounded inner lip ending in a simple point. The umbilicus is often completely covered with a thick and smooth callus.
The animal has a distinct lateral fringe of the foot, with three filaments on each side. The front of the right side near the base of the tentacles is produced into a fleshy lobe. The right tentacle is free, with the eye-peduncle compressed, and bears a rudimentary eye. The left eye-peduncle is cylindrical, with a distinct eye, and furnished with an expansion or frontal lobe, which is folded on itself and fringed at its free margin. The operculum is horny, orbicular, and multispiral.
The genus includes numerous species, such as Umbonium callosum, Umbonium conicum, Umbonium costatum, Umbonium elegans, Umbonium eloiseae, Umbonium giganteum, Umbonium moniliferum, Umbonium sagittatum, Umbonium suturale, Umbonium thomasi, and Umbonium vestiarium. Several species have been brought into synonymy, including Umbonium adamsi (synonym of Umbonium thomasi) and Umbonium depressum (synonym of Umbonium vestiarium).
Reader's Guide
Umbonium is notable as a genus within the family Trochidae, commonly called button top shells due to their distinctive low-spired, button-like shell shape. The shell's polished, porcellaneous exterior and thin pearly interior layer are characteristic features. The animal's anatomy includes a unique lateral fringe on the foot with three filaments on each side, and asymmetrical eye structures: the right eye is rudimentary while the left eye is distinct and associated with a fringed frontal lobe. The operculum is horny, orbicular, and multispiral. The genus contains a number of valid species, and many former species have been reclassified as synonyms, reflecting taxonomic revisions. The species list includes both historically described taxa and more recently named ones, such as Umbonium eloiseae. The genus is referenced in works by Link (1807-1808) and in molecular systematic studies by Williams, Karube & Ozawa (2008) that redefined the Trochidae family.
Did You Know?
- The shell of Umbonium is imperforated, meaning it lacks an open umbilicus, which is often covered by a thick callus.
- The animal has three filaments on each side of a distinct lateral fringe of the foot.
- Several species once placed in Umbonium, such as Umbonium adamsi, are now considered synonyms of other species.
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