Rissoina dorbignyi: Shell length up to 18.5 mm (N = 23); solid, strongly elongate, subcylindrical. Protoconch of 1 1/4, whorls; probably of planktotrophic larval type; with 2 weak spiral threads (observed in one fresh specimen from Fiji); transition to teleoconch abrupt, with a moderately deep sinusigera notch; margin weakly thickened. Teleoconch: of about 8-10 whorls; adapical spire whorls weakly to strongly convex, angulate below suture; sutures deeply impressed to subcanaliculated; abapical spire whorls and last whorl very weakly convex with sutures deeply impressed but the latter less canaliculate than on early spire whorls. Sculpture of 5-6 adapical spire whorls consisting of more or less prominent, rounded, closely spaced and slightly opisthocline axial ribs, crossed by moderately prominent spiral threads to spiral riblets, the latter forming weak nodules on the axial ribs; axial ribs gradually less prominent to absent on subsequent spire whorls and on last whorl; spiral ribs gradually less prominent, more numerous and more crowded on subsequent spire whorls; last whorl with very densely spaced, spiral threads. Aperture : D-shaped, outer lip weakly expanded; inner lip thin; columellar side weakly concave; anterior channel very shallow to almost absent; outer lip very thin, with a very weak and narrow external varix; outer lip weakly opisthocline. Colour: white throughout; some specimens covered with a brown-blackish layer of an unidentified substance. Operculum : typical of genus, with opercular peg open over all of its length (PONDER, 1985 : fig. 131 D). Radula : lateral teeth with ca. 5 cusps on its outer margin and 4-5 cusps on its inner margin.

Shell length up to 18.5 mm (N = 23); solid, strongly elongate, subcylindrical.
Protoconch: of 1 1/4, whorls; probably of planktotrophic larval type; with 2 weak spiral threads (observed in one fresh specimen from Fiji); transition to teleoconch abrupt, with a moderately deep sinusigera notch; margin weakly thickened. Teleoconch: of about 8-10 whorls; adapical spire whorls weakly to strongly convex, angulate below suture; sutures deeply impressed to subcanaliculated; abapical spire whorls and last whorl very weakly convex with sutures deeply impressed but the latter less canaliculate than on early spire whorls. Sculpture of 5-6 adapical spire whorls consisting of more or less prominent, rounded, closely spaced and slightly opisthocline axial ribs, crossed by moderately prominent spiral threads to spiral riblets, the latter forming weak nodules on the axial ribs; axial ribs gradually less prominent to absent on subsequent spire whorls and on last whorl; spiral ribs gradually less prominent, more numerous and more crowded on sub¬sequent spire whorls; last whorl with very densely spaced, spiral threads. Aperture : D-shaped, outer lip weakly expanded; inner lip thin; columellar side weakly concave; anterior channel very shallow to almost absent; outer lip very thin, with a very weak and narrow external varix; outer lip weakly opisthocline. Colour: white throughout; some specimens covered with a brown-blackish layer of an unidentified substance. Operculum : typical of genus, with opercular peg open over all of its length (PONDER, 1985 : fig. 131 D). Rissoina (Moerchiella) dorbignyi shows considerable variation with respect to the shape of the sutures of the adapical spire whorls, which can be deeply impressed to strongly canaliculated. Furthermore, there is some variation in the relative strength of the spiral sculpture on the adapical spiral whorls, but in other respects, this species appears to be rather uniform in both shell shape and sculpture.

Rissoina dorbignyi: Rissoina (Moerchiella) dorbignyi shows considerable variation with respect to the shape of the sutures of the adapical spire whorls, which can be deeply impressed to strongly canaliculated. Furthermore, there is some variation in the relative strength of the spiral sculpture on the adapical spiral whorls, but in other respects, this species appears to be rather uniform in both shell shape and sculpture.
The shell of Rissoina (Moerchiella) dorbignyi strongly resembles R. (M.) antoni and is contrasted under the affinities and differences of that species. The shell of R. (M.) dorbignyi strongly resembles R. (M.) striolata, but differs from the latter in having a paucispiral protoconch, and in having nodose axial ribs on the adapical spire whorls.
The shell of R. (M.) dorbignyi has a superficial resemblance to R. (M.) artensis but differs in having nodose, rounded, rather closely spaced axial ribs on the adapical spire whorls, rather than sharp, distantly spaced axial ribs in R. (M.) artensis; furthermore, the adapical spire whorls are much more angulated in R. (M.) artensis and the shell base has a weak to moderately prominent spiral cord, which is absent in R. (M.) dorbignyi. The shell of R. (M.) artensis is thinner and more fragile than the shell of R. (M.) dorbignyi and usually, R. (M.) dorbignyi is larger than R. (M.) artensis.
R. (M.) dorbignyi also resembles R. (M.) kilburni in shell characters and is contrasted where the latter species is introduced.

The shell of Rissoina (Moerchiella) dorbignyi strongly resembles R. (M.) antoni and is contrasted under the affinities and differences of that species. The shell of R. (M.) dorbignyi strongly resembles R. (M.) striolata, but differs from the latter in having a paucispiral protoconch, and in having nodose axial ribs on the adapical spire whorls.
The shell of R. (M.) dorbignyi has a superficial resem¬blance to R. (M.) artensis but differs in having nodose, rounded, rather closely spaced axial ribs on the adapical spire whorls, rather than sharp, distantly spaced axial ribs in R. (M.) artensis; furthermore, the adapical spire whorls are much more angulated in R. (M.) artensis and the shell base has a weak to moderately prominent spiral cord, which is absent in R. (M.) dorbignyi. The shell of R. (M.) artensis is thinner and more fragile than the shell of R. (M.) dorbignyi and usually, R. (M.) dorbignyi is larger than R. (M.) artensis.
R. (M.) dorbignyi also resembles R. (M.) kilburni in shell characters and is contrasted where the lat¬ter species is introduced.

Rissoina dorbignyi: Tropical Indo-West Pacific from the Red Sea to Fiji; so far, no specimens were recorded from the area between the Seychelles and Thailand, but likely this disjunct distribution is the result of poor collecting in the intervening areas. The presence of this species in the Mediterranean is probably due to recent immigration from the Red Sea through the Suez Canal.

Tropical Indo-West Pacific from the Red Sea to Fiji; so far, no specimens were recorded from the area between the Seychelles and Thailand, but likely this disjunct distribution is the result of poor collecting in the intervening areas (Fig. 53 C). The presence of this species in the Mediterranean is probably due to recent immigration from the Red Sea through the Suez Canal (BOGI e.a., 1984).