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Abstract
Two closely related species of Trachycardiinae from the Indo-West Pacific belonging to the Vasticardium orbita complex are revised. Young specimens are very similar in shell morphology but adult shells differ in general shape and rib structure. Based on these ontogenetic changes, and also on rib count, which appears to be a stable character, two species arc recognised: (1) Vasticardium orbita_(Broderip & Sowerby, 1833), with a mainly Pacific distribution and four subspecies : orbita, hawaiiensis, mendanaense, and philippinense and (2) Vasticardium luteomarginatum_(Voskuil and Onverwagt, 1991), with a primarily Indian Ocean distribution and three subspecies: luteomarginatum, marerubrum, and insulare, the last of which is herein described as new.
The two species are usually easy to separate, but some specimens are difficult to differentiate. The apparently allopatric distribution of the two species raises the possibility that a single species , with discrete geographical subspecies, is involved.