Plectostoma pumilio (Smith, 1894)
“This very small species closely resembles O. austeni in its general features. It is, however, quite distinct. It consists of one whorl less, is of smaller size, has rather more distant lirae or lamellae and also a more obtuse or less conical spire. The coloration is the same as in that species.” (E. A. Smith, 1894)
E. A. Smith (1894) original descriptions on Opisthostoma pumilio – “Testa O. austeni similis, sed minor, minus conica vel pyramidalis; anfractus 5, perconvexi, liris obliquis leviter distantioribus instructi.”
Kobelt (1902) Deutsch descriptions on Opisthostoma (Plectostoma) pumilio – “Schale der von O. (P.) austeni ähnlich, aber kleiner, weniger ausgesprochen kegelförmig, die schrägen Eeifen etwas weitläufiger; 5 sehr gewölbte Windungen.”
By Vermeulen (1994)
"Spire conical with distinctly convex sides. Apex oblique. Whorls 3 7/8-4 5/8, convex; last whorl rounded. Constriction with a transverse palatalis. Tuba touching the spire over its entire length,
gradually narrowed towards the constriction, rounded below. Radial ribs (moderately) spaced (3-7
ribs/0.5 mm on the penultimate whorl and half-way the tuba), on the spire slightly sinuous, but those on
the last whorl not or hardly sinuous; those on the tuba not sinuous below, sometimes with some much
finer radial riblets in between. Spiral striation absent. Umbilicus open, 0.10-0.15 mm across. Aperture
tilted up to 30 degree with regard to the coiling axis, its upper margin widely below the level of the apex, subtriangular with an obtusely angular lower edge and a slightly wider rounded upper and right edge, the left side well rounded. Peristome (almost) touching the spire, double; outer peristome
spreading slightly beyond the inner, but gradually narrowed towards, and absent along the right side
and often along the lower side of the aperture, often slightly widened along the upper side; inner
peristome moderately protruding from the outer, not drawn out over the spire on the right side of the
aperture, sometimes slightly spreading. Spire: height 1.3-1.7 mm; width 1.1-1.2 mm; index 1.2-1.4.
Total width 1.8-2.1 mm. Height aperture 0.6-0.7 mm; width 0.5 mm
Opisthostoma (Plectostoma) pumilio – “H. 2, Durchm. 2.25 mm.” (Kobelt, 1902)
Shell height – 2 mm; Shell width – 1.25-2.25 mm. Aperture diameter – 0.75 mm.
Type locality – “Rumbang, Sarawak” (E. A. Smith, 1894) (1.268388,110.263839)
Limestone hill 7.5km WNW of Kpg. Piching, near Serian. Sarawak, Malaysia.