Shell up to 2,65 mm wide, depressed-skeneiform, spire slightly raised or perfectly flat, thin, fragile, colourless, transparent, umbilicate. Periostracum thin, smooth and colourless. Protoconch of about 1,2 convex whorls, 350-300 µm wide, tip of apical fold evenly rounded, last quarter whorl rapidly expanding. Tip of apical fold very finely maleated against suture, elsewhere smooth. Apertural rim shallowly prosocyrt, rim a fine, crisply defined, thread-like varix. Teleoconch of up to 1,75 regularly expanding whorls that are evenly convex from suture into umbilicus. Umbilical wall with a very finely and sharply roughened surface and obscurely spirally lirate, shell elsewhere smooth and polished, with faint collabral growth lines. Umbilicus perspective to protoconch, diameter 20,3-22,6% of adult shell diameter (2,30-2,60 mm). Aperture subcircular, profile prosocline. Peristome thin, parietal contact area narrow. No basal notch.Animal . Snout depressed, almost parallel-sided, tip concave in front. Cephalic tentacles flattened, long and narrow, tapered. Foot deeply bilobate anteriorly. Right suboptic tentacle slender, about as long as cephalic tentacles. No eyes, optic cups, or epipodial tentacles. Opercular lobe thin and spreading, operculum rather loosely attached, typical.
Radula with the formula oo+6+1+6+oo. Cutting areas of central and lateral teeth similar, long and narrow, each gradually tapered to a sharp point, edges of central tooth smooth, edges of lateral teeth smooth apart from a few fine serrations on inner proximal edge. Central and lateral tooth shafts grooved to interlock with adjacent teeth. Innermost few marginal teeth little longer than laterals, lengthening and narrowing outwards. Cutting area of innermost tooth with a strong distal cusp and finely serrate outer edge. Cutting areas of each outer marginal tooth very long and almost parallel-sided, with numerous long, narrow cusps along outer edge and on rounded tip, finer cusps along inner edge.
Known distribution. White Island to Cape Palliser, New Zealand, 1037-1100 m.
Remarks. Hyalogyra expansa is immediately separable from other species described herein by its low, glassy, essentially smooth shell, and wide umbilicus. Among other species with which it may be meaningfully compared, H. expansa superficially resembles the New Zealand 'Zalipais' benthicola Powell, 1927 (Skeneidae?) and 'Cochliolepis' albiceratus Ponder, 1966 (Vitrinellidae?), the shells of which differ, however in several respects, notable in being smaller relative to the number of whorls. While these latter species certainly do not belong to the genera in which they currently reside, it is inappropriate to discuss them further here.