Plectostoma praeco (van Benthem Jutting, 1961)

Benthem-Jutting (1961) original descriptions on Opisthostoma (Plectostoma) praeco – “Shell with regular, broad-conical spire, ending in an abnormally coiled and detached last whorl, with a trumpet-shaped aperture. At the transition of the regular spire into the abnormal terminal part the whorl is transversely constricted. White or very light-pink, shing and somewhat transparent. First 1 ½ whorls smooth, following ones ornamented with delicate, oblique, transverse, white ribs, standing away wing-like from the shell. There are 30 ribs on the last normal whorl before the constriction. After the constriction the ribs stand wider apart. In the holotype there are 12 ribs between the constriction and the aperture, a distance of 2 ½ mm. There is a delicate spiral striation. Whorls 6 (the detached trumpet not included), convex. Suture deep. The convexity is accentuated by the wing-like ribs. Top pointed, but not sharp. Base rounded. Umbilicus distinct and deep, but not wide. Last whorl irregular, transversely constricted in the middle, then widening towards the aperture, at the same time turning back and a little upward, free from the spire. Aperture facing obliquely up and back, thus falsely suggesting a sinistral shell. Aperture round. Peristome continuous, circular. Just behind the peristome, and parallel with it the terminal part of the trumpet bears a wing-like collar. Operculum unknown.”

The shell is small, white or pinkish white, shining and somewhat transparent with broad-conical spire and circular in cross-section. Whorls six (tuba is not included), convex with deep suture. The first one and half whorl are smooth, and following whorls ornamented with thin, oblique, transverse, white, and wing-like ribs which and widely spaced (four ribs/0.5 mm on the penultimate whorl and five ribs/0.5 mm on the body whorl close to the constriction). Umbilicus is narrow and approximately 0.25 mm across. The body whorl is irregular and constricted with transverse palatalis. The tuba is completely detached after the six whorls, turning back and projecting a little upward. Aperture is ovate with continuous and double rounded peristome which is detached from the spire. Operculum is unknown. The shell height is between 2.4-2.0 mm, the total width is between 3.0-2.8 mm, and aperture diameter is between 1.2-1.0 mm.