Diagnosis. — Constriction with a distinct parietalis which may continue up to the sharp inner curve of the tuba and which is equally high over its entire length, or highest near the constriction, or which may be much shorter and inconspicuous, without an angularis. Spire with 4-14 ribs/0.5 mm on the penultimate whorl; tuba with 3-7 ribs/0.5 mm half-way. Umbilicus 0.05-0.25 mm wide.
Distribution. — SARAWAK. 1st Div., limestone ranges S. of Kuching, rare and on scattered localities only. 4th Div: common in limestone areas.
O. brachyacrum can be divided into three subspecies mainly on account of the configuration of the teeth in the constriction. Subspec. tatauense is well characterized by a differently shaped longitudinal parietalis. Subspec. lambii differs from subspec. brachyacrum in the presence of a short angularis, but within both subspecies specimens occur in which one or more teeth in the constriction are inconspicuous or absent. Only in the limestone ranges S. of Kuching, where subspec. brachyacrum and subspec. lambii occur together, such specimens hardly obscure the difference between them. In other limestone areas within their combined range either one, or the other subspecies is found,
or series of specimens morphologically close to one of the two subspecies. In some limestone areas they are found together with series of specimens in which the diagnostic teeth in the constriction are (partly) missing, so that the unequivocal identification of individual specimens is not always possible. These are provisionally included in the subspecies with which they occur sympatrically.