''Oxystele impervia'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

sea snail Sea snails are slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the Taxonomic classification, taxonomic class Gastropoda with slugs, which are distinguishe ...

, a marine

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Trochidae The Trochidae, common name top-snails or top-shells, are a family of various sized sea snails, marine gastropod molluscs in the subclass Vetigastropoda. This family is commonly known as the top-snails because in many species the shell resembles ...

, the top snails.

Description

The size of the shell varies between 12 mm and 18 mm. The imperforate, rather thin shell has a conoidal shape. Its ground color is (usually) whitish, pinkish or bluish, painted with red, brown or blackish in innumerable patterns, but usually in one of the following modes : * longitudinal zigzag dark stripes on a light clear ground * very obliquely descending narrow stripes or lines, generally zigzag or interrupted about the middle of the

whorl A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). In nature File:Photograph and axial plane floral diagra ...

* narrow spiral articulated bands around the middle of the whorl, the base mottled or barred, and the suture bordered by a row of dark blotches * ground-color pink or purplish, the entire surface variegated by rather narrow spiral bands, finely articulated with red or purple and white The elevated

spire A spire is a tall, slender, pointed structure on top of a roof of a building or tower, especially at the summit of church steeples. A spire may have a square, circular, or polygonal plan, with a roughly conical or pyramidal shape. Spire ...

is conical. The sutures are distinctly impressed. The

apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics) A-Bomb Abomination Absorbing Man Abraxas Abyss Abyss is the name of two characters appearing in Ameri ...

is minute and acute when not eroded. The 6 whorls are convex, quite smooth outside, but when eroded the

nacre Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is ...

is seen to be finely spirally lirate. The base of the shell is not eroded. The

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

is very oblique. The acute outer lip is narrowly margined, iridescent within, the nacre smooth, but apparently lirate. The white, thin

columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...

is arcuate, concave. It covers the place of the umbilicus with a pad of callus, the outer edge of which is usually not appressed closely to the base. The parietal wall contains a thin translucent callus or none. The umbilico-columellar callus is bounded outside by a more or less obvious streak of blue, green, brown, or sometimes light yellowish.Tryon (1889), Manual of Conchology XI, Academy of Natural Sciences, Philadelphia
(described as ''Monodonta sagittifera'' )

Distribution

This marine species occurs off the southern Atlantic coast of

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

References

* Steyn, D.G. & Lussi, M. (1998) ''Marine Shells of South Africa. An Illustrated Collector’s Guide to Beached Shells''. Ekogilde Publishers, Hartebeespoort, South Africa, ii + 264 pp. page(s): 24
Branch, G.M. et al. (2002). ''Two Oceans''. 5th impression. David Philip, Cate Town & Johannesburg.
* Donald K.M., Kennedy M. & Spencer H.G. (2005) ''The phylogeny and taxonomy of austral monodontine topshells (Mollusca: Gastropoda: Trochidae), inferred from DNA sequences''. Molecular Phylogenetics and Evolution 37: 474–483. {{DEFAULTSORT:Oxystele Impervia impervia Gastropods described in 1843