Plectostoma cookei (Smith, 1894)  

Smith (1894)'s description

"Testa conica, perforata, pallide rufescens; spira subtiliter concave conica, ad apicem obtusiuscula; anfractus 6-7, superiores convexi, laeves, inferiores minus convexi, oblique et tenuissime lamellati, ultimus ad peripheriam obtuse angulatus, antice constrictus, retroversus, ascendens, conspicue solutus, lamellis validioribus distantioribus bicristatis ornatus; apertura rotundata, intus rufescens, ad marginem duplicatam expansam pallida, usque ad suturam tertiam vel quartam sursum porrecta. longit. 2 millim.; diam. maj. 2 2/3. min. 1 1/2."

Vermeulen (1994)'s description

"Spire conical with flat to distinctly concave sides. Apex not oblique. Whorls 5 5/8 - 6 3/8; top whorls convex; next whorls slightly convex; last whorl obtusely angular at the periphery, flat above, slightly convex below. Constriction with an inconspicuous parietalis, a transverse palatalis, a distinct columellaris. Tuba gradually narrowed towards the constriction, rounded, somewhat angular or with a slight, obtuse ridge below. Radial ribs on the spire (rather) closely placed (5-6 ribs/0.5 mm on the penultimate whorl), hardly visible but on the lower third of the whorls with a slight, shallowly concave projection abrading to a sinuous scar, those close to the tuba similar but abrading to a scar with a single, shallow or deep loop when the shell is observed in front view; those on the tuba moderately to widely spaced (1-3 ribs/0.5 mm half-way), more distinct, below with a short, (deeply) trough-shaped projection abrading to a sinuous to deeply looped scar. Spiral striation absent. umbilicus open, 0.35-0.40 mm across. Aperture tilted up to 30 degree with regard to the coiling axis, its upper margin widely below the level of the apex, circular to elliptic. Peristome widely distant from the spire, double; outer peristome hardly to moderately spreading beyond the inner, but gradually narrowed towards, and absent along the right side of the aperture; inner peristome slightly protruding from the outer, widely spreading. Spire: height 2.0-2.5 mm; width 1.3-1.6 mm; index 1.5-1.6. Total width 2.8-3.0 mm. Height aperture 0.7 mm; width 0.6-0.7 mm."