Retusa, Brown, 1827

Valdés, Ángel, 2008, Deep-sea “ cephalaspidean ” heterobranchs (Gastropoda) from the tropical southwest Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 587-792 : 727-728

Genus RETUSA Brown, 1827

Retusa Brown, 1827 : pl. 38. Type species: Bulla obtusa Montagu, 1803 [Recent, northeast Atlantic and Mediterranean], by monotypy.

Synonyms:

Coleophysis Fischer, 1883 [in 1880-87]: 555. Type species: Bulla truncatula Bruguière, 1792 [Recent, northeast Atlantic and Mediterranean], by monotypy.

Cylichnina Monterosato, 1884: 143 . Type species Cylichna laevisculpta Granata, 1877 [Recent, Mediterranean], by subsequent designation of Crosse (1885).

Mamilloretusa Nordsieck, 1972: 36 . Type species Bulla mammillata Philippi, 1836 [Recent, Mediterranean], by original designation.

DIAGNOSIS. — Shell morphology. Shell external, fragile, with parallel or convex sides; spire visible or not. Aperture longitudinal, narrow and anteriorly rounded. Inner lip thickened. Columella simple, with no folds. Operculum absent.

Anatomy. Large headshield, with the posterior end divided into 2 projecting lobes. Radula absent. Gizzard with 3 lobate plates.

REMARKS. — The genus Retusa was introduced by Brown (1827) for Bulla obtusa , a species with a distinct spire of variable height. Thompson (1976) also included in Retusa the species Retusa truncatula (Bruguière, 1792) and Retusa umbilicata (Montagu, 1803) , the type species of Coleophysis Fischer, 1883 and Cylichnina Monterosato, 1884 , respectively. Retusa truncatula is characterized by having a truncate and umbilicate apex, whereas in R. umbilicata the aperture bends over the apex of the shell and the parietal wall forms a short prolongation that covers part of the apex. There are no anatomical differences that justify distinguishing three genera. Species belonging to these three forms of Retusa are treated below.

Mamilloretusa was introduced by Nordsieck (1972) for species of Retusidae with the protoconch protruding from the apex of the shell. However, Thompson (1976) showed that within the species Retusa obtusa and Retusa truncatula some specimens have a protruding spire and/or protoconch, whereas in others the spire and/or the protoconch are embedded in the apex.