About Diloma nigerrimum (Gmelin, 1791)
This species is Diloma nigerrimum (Gmelin, 1791), a marine gastropod. The shell reaches a height of 24 mm and a diameter of 26 mm. It is solid, black, imperforate, depressed, and has a globose shape. Its shell sculpture is made up of numerous close spiral striae, which are sometimes nearly obsolete. These striae are crossed by oblique, often strongly developed growth lines. The shell is naturally bluish-black or solid black; beach-worn specimens may appear reddish or brownish, and the shell has no spotting. The epidermis is fairly thick and solid, and is shiny in fresh specimens. The spire is short, ranging from conoidal to conical in shape, with an apex that can be rounded or acute. The protoconch holds two spirally striate, lightly pearly whorls that are sometimes reddish. The shell has 4 to 5 total whorls, which are slightly convex and increase in size rapidly. The body whorl is usually depressed or slightly concave below the suture. The shell's base is rounded, eroded, and iridescent in front of the aperture. The suture is linear, with a strong cord forming a margin along its lower edge. The large aperture is oblique, greenish iridescent, and marked with closely spaced lirate ridges. The outer lip is convex, rather thin and sharp, bordered on the inside by an extremely narrow black margin. This black margin is followed by a broad opaque white band that is sometimes brilliantly iridescent. The columella is concave, faintly subdentate near its lower end, very broad, and flattened or excavated across its face. It is formed mostly of an opaque white layer that also lines the shell's base, but this layer does not reach the edge of the lip. The parietal wall has a nacre band that joins the two ends of the peristome, and it is bounded on the outside by light brown or white. While the dentition of the New Zealand morph of this species is quite different from that of the South American D. nigerrimum morph, genetic analysis confirms that both morphs belong to the same species. This marine species is endemic to New Zealand and the west coast of South America.
