Plectostoma H. Adams 1865

Plectostoma is a genus of air-breathing land snails in the family Diplommatinidae.

Apex. Protoconch is either slightly, moderately or distinctly convex (Figure 3). Spire. Height: 1.0 mm–3.7 mm. Width: 0.85 mm–2.60 mm (Figure 11). Number of whorls between 2 3/4–71/4. Apical spire shape: oblong or depressed conical (Figure 4). Basal spire shape: conical, ovoid or ellipsoid (Figure 5). Whorl periphery: flat, slightly, moderately or distinctly convex. Umbilicus: open, partially closed, or totally closed. Constriction. Parietal teeth: parietal side of inner constriction whorl (Figure 2) with two long lamellae (Figure 6A), two ridges with a knob at each end (Figure 6B), one ridge with a knob at one end (Figure 6C), or no teeth (Figure 6D). Basal teeth: basal side of inner constriction whorl (Figure 2) with no teeth (Figure 7B), one ridge running parallel with the whorl growing direction, one ridge with a knob at one end running perpendicular to the whorl growing direction, or a combination of the latter two types (Figure 7A). Tuba. Coiling direction: regular coiling (type 1, Figure 8A) or distorted (Type 2, or 3) (Figure 8B and C). Tuba whorl length in proportion to spire last whorl: ca. 3/8–1 1/2. Proportion of tuba that attaches to spire: whole to none. Aperture and peristome. Peristome: simple aperture without outer peristome (Figure 10B), or double peristome (Figure 10C and 10D). Shape of outer peristome (Figure 10A): same as inner peristome and uniformly round, or highly projected or slightly projected at either a particular side or at a several sides of anterior, poteriorior, left and right laterial (Figure 9 and 10A). Spiral lines. Either thick or thin, or only thin lines present (Figure 12). Radial ribs. Rib density: 4–32 per mm on the spire’s last whorl in right lateral view (Figure 13A). Intensity: thick or thin (Figure 13B). Shape: straight, slightly curved, distinctly curved, single humped, single looped or double looped and the shape remaining the same or changing between between the spire and the tuba (Figure 13C, but single-looped, and double-humped not shown). Inclination: from orthoclin to prosoclin.