Opisthostoma brachyacrum brachyacrum (Thompson, 1978)
“A species of the subgenus Opisthostoma with a short raised apex and a recurved body whorl that ascends the spire to the shoulder of the penultimate whorl. The aperture faces posteriorly and bears a double peristome, the outer being only moderately wider than the inner. The sculpture consists of sparse feeble axial ribs, and very thin raised spiral threads.” (Thompson, 1978)
Thompson (1978) original descriptions on Opisthostoma (Opisthostoma) brachyacrum – “Shell minute, about 0.9 mm high and slightly wider in major diameter, with about 4 whorls. The umbilicus is open and circular and is about 0.2 times the minor diameter of the base. The spire is abbreviate cylindrical with a shortly elevated spex that consists of about 2 whorls. The embryonic with a shortly elevated apex that consists of about 2 whorls. The embryonic shell consists of 1.5 whorls that are distorted abliquely to the shell axis. The shell is widest at the third (penultimate) whorl and is slightly narrower below. The body whorl is slightly constricted below the aperture and then enlarges again just behind the peristome. The body whorl is strongly recurved upward and backward along its last quarter and ascends the spire to the shoulder of the penultimate whorl againt which the recurved portion is appressed. The embryonic whorls are sculptured with minute granules. The postembryonic whorls are sculptured with minute granules. The postembryonic whorls bear very low widely spaced axial ribs. The ribs are nearly uniform in height and width over the surface of the shell and continue into the umbilicus. There are 23 ribs on the penultimate whorl. Ribbing is absent in the vicinity of the constriction on the body whorl. Fine, delicate, spiral threads are present between the ribs on the last part of the body whorl but are not obvious elsewhere, perhaps dua to the worn condition of the holotype. The aperture is circular and is appressed against the penultimate whorl. It lies in a plae posteriorly slightly oblique to the shell axis and bears a double peristome. The outer peristome consists of several fused lamella. It is slightly wider than the inner peristome and is nearly uniformly expanded. The inner peristome is trumpet-shaped and is nearly equally expanded around the aperture. The parietal wall of the inner peristome is appressed against the penultimate whorl and extends forward at that point.”
Description by Vermeulen (1994)
"Shell white, slightly translucent. Whorls 3 1/4 - 3 1/2 (tuba not included); top whorls slightly to moderately oblique, moderately elevated; body whorls forming an usually slightly obliquely cylindrical body, convex, well rounded, with the suture well impressed. Constriction rather distinct, gradual. Tuba consisting of approximately 1/2 whorl (rarely somewhat less), touching the body whorls over its entire length, approximately circular in section. Shell with narrow, usually inconspicuous radial ribs, which are widely spaced near top, moderately spaced and low on the upper body whorl (23-36 ribs present on this whorl), approximately similar but more widely spaced on the lower body whorl, widely spaced or almost absent over the area of the constriction, and moderately spaced and often slightly more prominent on the tuba. Ribs crossed by fine spiral striation. Umbilicus open, deep, narrow. Aperture with the imaginary plane through the inner peristome tilted up to 30 degrees with regard tio the coiling axis of the body whorls; with the upper margin not protruding beyond the plane perpendicular to the coiling axis of the body whorls as well as through the apex; widely ovate to circular; apertural teeth absent; inner peristome moderately thickened, moderately reflected; outer peristome moderately flaring on the palatal side. Constriction inside with two teeth: one longitudinal, long, lamella-like parietalis, starting at the constriction and projecting into the tuba, but ending well before reaching the peristome (the end not visible deep inside the aperture); one distinct, oblique, rather high infracolumellaris which projects as a distinct, transverse ridge over the palatal surface (mising in some specimens). Height of body whorls 0.8-1.0 mm; width including tuba 1.2-1.4 mm, without tuba 0.6-0.8 mm; height of aperture 0.4-0.5 mm; width of aperture 0.4-0.6 mm."
“Measurement of the unique holotype: height of shell, 0.93 mm; major width, 1.16 mm; minor width 0.72; aperture width, 0.46 mm; aperture height, 0.46 mm.” (Thompson, 1978)
Type locality – “Borneo; Sarawak; Fourth Division; limestone hill on the trail from the Niah River to Niah Cave, Batu Niah” leg. W. I. Holzmark/Nov. 4, 1976 (Thompson, 1978)