''Emarginula retrogyra'' 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 ...

Fissurellidae Fissurellidae, common name the keyhole limpets and slit limpets, is a taxonomic family of small to medium-sized limpet-like sea snails, marine gastropod molluscs in the clade Vetigastropoda.Rosenberg, G. (2012). Fissurellidae. Accessed throu ...

, the keyhole limpets and slit limpets.

Description

(Original description) Dimensions: Holotype, base 10.5 × 8.1 mm, height 6.7 mm (= largest specimen). Shell: Small and thin (basal length up to 10.5 mm); relatively high (H/L ± 0.64). The anterior slope is convex, but not strongly so and 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 not extending over the posterior margin of the base. The tip of the apex is nonetheless strongly recurved. The basal outline is evenly ovate (Length/Depth ± 1.3), slightly concave in side view. The anterior face is not flattened. The posterior slope is for the most part straight, becoming strongly concave beneath the recurved apex. The

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

is tucked under the recurved apex, but largely remains free of the ascending posterior face of

teleoconch The gastropod shell is part of the body of many gastropods, including snails, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium ...

, due to a strong apical recurvature. The anterior slit is moderately deep, approx. one-third of the anterior slope. The margins of the

selenizone A selenizone (from the Greek "selene" meaning "moon", and "zone" meaning " girdle") is an anatomical structure that exists in the shells of some families of living sea snails: the slit shells, the little slit shells and the abalone, which are ...

are distinctly elevated, but thin and mostly broken away in the holotype, but partially remaining in paratypes. The selenizone itself is not raised, bearing coarse, evenly-spaced lunulae. These become more numerous and more close-set nearer to the slit. The remaining

sculpture Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...

consists of fine, close-set radial ribs, of which approx. 22 are primary with secondary and tertiary intermediaries between them. The primary and secondary ribs are more or less equal in size at the shell margin, tertiaries are usually slightly weaker. The intervals between the ribs are narrow, but well-defined. The concentric sculpture consists of fine, close-set ridges. These are weaker and not as markedly raised as the radial ribs. The ridges cross the ribs rendering the latter crisply nodular. The interstices between ribs and ridges are deep, quadrate, simple, with one or a pair of intritacalx pits. The basal margin is mostly damaged in the holotype, but evidently finely crenulated at ends of radial ribs with shallow grooves on the interior, underlying radial ribs, longer and more evident in juveniles. The interior shows a broad ridge underlying the selenizone, approx. twice the length of the slit. The ridge itself has a shallow central furrow. The interior of the apex is scarcely visible due to extent of recurvature.

Protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

: Typically emarginuliform. It consists of a single whorl with thickened terminal

varix A varix (: varices) is an abnormally dilated blood vessel with a tortuous course. Varices usually occur in the venous system, but may also occur in arterial or lymphatic vessels. Examples of varices include: * Varicose veins, large tortuous veins ...

; rotated 90° further with regard to teleoconch such that varix is parallel with the plane of the shell aperture; surface with sparse superficial granulation; maximum diameter ± 180 μm. Colour:

Holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

rather dirty pale olive-brown, but fresher specimens show broad, darker greenish-brown concentric bands and occasional greenish-brown spots.

Distribution

This marine species occurs on the

Walters Shoals The Walters Shoals is a group of submerged mountains off the coast of Madagascar. The shoals are south of Cape Sainte Marie - Madagascar and east of Richards Bay at the African coast. The tips of some of the mountains are only below the surf ...

, a group of submerged mountains off the coast of

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

References

{{Taxonbar, from=Q125202416 retrogyra Gastropods described in 2024