Perotrochus caledonicus: Description of holotype. Shell trochoid, grossly equilateral in outline, thin and light in structure, composed of 8.3 whorls with conspicuous spiral ornamentation. Spire coeloconoid; apical angle of first 5 whorls 57°; mean spire angle 72°. Protoconch (940 µm) a bulbous nucleus followed by 0.3 of a whorl which is smooth, glassy, and weakly delimited from teleoconch. Protoconch followed by a white, finely granular whorl.
Teleoconch sculptured by spiral cords crossed by-finer collabral incremental lines, giving first 4 whorls a beaded appearance readily visible under the microscope. This appearance weakening on succeeding whorls as axial riblets become obsolete. Surface between growth riblets sculptured by fine, radiating threads. Early postlarval sculpture consisting of axial lines above and below selenizone, in conjunction with suture and selenizone forming squarish to rectangular spaces. Axial lines each forming a knob just above suture in lower part of whorl; above selenizone, axial lines gradually becoming interrupted by a spiral keel, the two forming a knob where they intersect. Additional spiral keels later appearing both above and below selenizone, together with 1 median keel; axial ribs concurrently becoming more obsolete. Sculpture from 5th whorl on predominantly spiral, with 2, 1, and 2 ribs respectively above, on, and below selenizone. Third spiral keels appearing on 6th whorl above selenizone and on 7th whorl below it.
Body whorl with 8 spiral cords above selenizone, 4 below, and 2 on fasciole; axial ribs obsolete here, but microsculpture of minute, diverging threads still present.
Profile of whorls nearly straight, but weakly convex on last 2 whorls, hence spire slightly concave; sutures scarcely impressed, very incon-spicuous. Selenizone in mid whorl in earlier part of shell, then gradually descending to lower two-thirds. Fasciole faintly concave, bordered by 2 inconspicu¬ous keels and bearing both microsculpture of diverging threads and curved incremental lines; these lines sharp in upper whorls, weakening near end of 5th whorl, then becoming obsolete. A white, beaded spiral cord appearing in selenizone of 3rd whorl, then strengthening progressively and at whorl 5.8 losing its beading. Space between spiral cord and lower edge of fasciole widening shortly before 7th whorl; a second white, spiral cord appearing in first part of 7th whorl, weaker than the first at level of open slit, but these cords dividing selenizone into 3 equal strips.
Aperture horizontally oblong, the part above the slit running about 0.1 of a whorl further than the lower part. Slit short, 20.5 mm along its upper margin, i.e., one-quarter the circumference of the last whorl. Columella and interior of aperture covered with a thin nacreous layer; base inside aperture with only a narrow band of nacre exposed, adjacent to suture. Colurnellar lip sharp, thin, with a moderate sigmoid flexure. Base almost flat, with a small, shallow umbilical depression; umbilicus almost completely closed, consisting of a tiny slit between base and nacreous colurnellar fold. Base dirty white, with a few faint growth lines not intersecting the 21 rounded spiral cords. Shell otherwise pale beige-orange mottled with brown-red, tending to crimson on final whorl and lower part of earlier whorls. Spiral cords unicolorous dirty white.
Operculum almost circular, multispiral; each whorl with a conspicuous free margin.

Perotrochus caledonicus: The « New Caledonia Slit Shell » is a small, lustrous pleurotomariid endemic to New Caledonia as its name suggests. For many years, this species was thought to occur in two forms with distinct sculpture a 'smooth form' with uninterrupted spiral cords and a 'pustulose form' with the cords divided into many conspicuous beads. Although no obvious intergrades were known, the two were similar in size, colouration and shape as well as co-occuring in similar areas and were thus assumed to represent a single species. A recent study using both morphological and molecular data clearly revealed, however, that the two in fact represent two distinct lineages and are separate species. The 'smooth form', as shown here, corresponds to the nominal M. caledonicus and is more common in the southern New Caledonia, though the distribution range does extend northwards up to the Grand Passage area. The 'pustulose form' was newly described as Perotrochus wareni Anseeuw, Puillandre, Utge & Bouchet, 2015; it is more common in the northern New Caledonia but also occurs in the south, and the range extends westwards to Lansdowne Plateau in the Coral Sea. These two species differ slightly in depth preference, with M. caledonicus occuring mostly around 400m deep and P. wareni mostly around 350m deep. The population from Chesterfield Plateau in the Coral Sea, previously also attributed to P. caledonicus, was revealed to be a further distinct species and was named P. pseudogranulosus Anseeuw, Puillandre, Utge & Bouchet, 2015. The name M. caledonicus therefore currently refers solely to the 'smooth form'. A carnivorous grazing gastropod, it lives on hard substrates and feeds on demosponges. Typical shell length around 40mm., extremely large specimens are known to exceed 65mm. Though it is locally not uncommon, high quality specimens are still uncommon in the international shell trade.

Perotrochus caledonicus: Finding Place: Southwest of the Isle of Pines, New Caledonia