Rissoina gigantea: Shell length up to 20 mm (N = 2), solid, narrowly conical.
Protoconch : broken in all specimens available. Teleoconch : of about 8, weakly convex whorls, weakly subangulated below the periphery; adapical spire whorls slightly more angulated than abapical whorls; last whorl weakly to moderately globose and moderately contracted near shell base. Adapical spire whorls with weak to very prominent, slighly opisthocline to weakly prosocline, rather narrow to moderately large, rounded axial ribs, gradually weaker and eventually absent on abapical spire whorls and on last whorl; spire whorls with 4-5, rather weak spiral riblets, crossing the top of the axial ribs and forming small to moderately prominent nodules on the latter; spiral ribs becoming gradually less prominent, gradually changing into more closely spaced threads on penultimate and last whorls. Aperture: D-shaped to lenticular; inner lip thin; anterior channel narrow, weakly to moderately deep; outer lip weakly thickened internally, with rather thick and narrow external varix; outer lip moderately to strongly opisthocline in profile. Colour: white throughout. Operculum, radula and internal anatomy : unknown.

Shell : length up to 20 mm (N = 2), solid, narrowly conical. Protoconch : broken in all specimens available. Teleoconch : of about 8, weakly convex whorls, weakly subangulated below the periphery; adapical spire whorls slightly more angulated than abapical whorls; last whorl weakly to moderately globose and modera¬tely contracted near shell base. Adapical spire whorls with weak to very prominent, slighly opisthocline to weakly prosocline, rather narrow to moderately large, rounded axial ribs, gradually weaker and eventually absent on abapical spire whorls and on last whorl; spire whorls with 4-5, rather weak spiral riblets, crossing the top of the axial ribs and forming small to moderately prominent nodules on the latter; spiral ribs becoming gradually less prominent, gradually changing into more closely spaced threads on penultimate and last whorls.
Aperture: D-shaped to lenticular; inner lip thin; ante¬rior channel narrow, weakly to moderately deep; outer lip weakly thickened internally, with rather thick and narrow external varix; outer lip moderately to strongly opisthocline in profile. Colour: white throughout. Operculum, radula and internal anatomy : unknown. Usually, the last whorl is strongly inflated, but in some specimens it is only very weakly globose; furthermore, the axial ribs on the adapical whorls can be more or less prominent.

Rissoina gigantea: Usually, the last whorl is strongly inflated, but in some specimens it is only very weakly globose; furthermore, the axial ribs on the adapical whorls can be more or less prominent. The shell of Rissoina (Moerchiella) gigantea superficially resembles that of R. (M.) striata from which it differs in being more elongated and less broadly conical, in having angulated whorls and in the last whorl being more inflated; furthermore, the axial and spiral sculpture is equally prominent on the adapical spire whorls of R. (M.) striata, while the axial sculpture is much more prominent than the spiral sculpture in R. (M.) gigantea.
R. (M.) gigantea resembles R. (M.) dorbignyi in shell characters, but differs in being usually much larger, in having more prominent axial ribs on the adapical spire whorls, in having less angulated adapical spire whorls, in having a shorter aperture and in the last whorl being much more inflated. Rissoina grandis is very probably conspecific with R. (M.) gigantea, but since the original description is rather vague and not accompanied by any illustration, and because the whereabouts of the type material of PHILIPPI is unknown, we prefer to treat R. grandis as a nomen dubium.

The shell of Rissoina (Moerchiella) gigantea super¬ficially resembles that of R. (M.) striata from which it differs in being more elongated and less broadly conical, in having angulated whorls and in the last whorl being more inflated; furthermore, the axial and spiral sculpture is equally prominent on the adapical spire whorls of R. (M.) striata, while the axial sculpture is much more prominent than the spiral sculpture in R. (M.) gigantea.
R. (M.) gigantea resembles R. (M.) dorbignyi in shell characters, but differs in being usually much larger, in having more prominent axial ribs on the adapical spire whorls, in having less angulated adapical spire whorls, in having a shorter aperture and in the last whorl being much more inflated.

Rissoina gigantea: Philippines, New Guinea and Solomon Islands, northern Australia and New Caledonia .

Philippines, New Guinea and Solomon Islands, northern Australia and New Caledonia