Shell: minute to small, ovate to conic, non-umbilicate to umbilicate, smooth or, typically, with axial ribs stronger than spiral sculpture; axial ribs usually obsolete on base. Aperture with simple peristome, inner lip narrow, attached to parietal wall, angled posteriorly, anterior end rounded, simple. Outer lip opisthocline to prosocline, with or without external varix. Protoconch of 1 1/2 to 2 1/4 whorls, smooth or with spiral threads or spiral rows of minute, closely spaced pustules.
Head-foot: cephalic tentacles long, with parallel sides, ciliated; foot constricted in middle, with posterior mucous gland. Anterior (left) and posterior (right) pallial tentacles present, or only posterior. A single, slender metapodial tentacle present. Exterior pigment patches present in some species (often black and yellow).
Operculum: simple, thin, nucleus eccentric, last whorl large.

North Atlantic and Mediterranean Sea, Japan, tropical western Pacific, South Africa and Australasia. Paleocene to Recent.

Species of Pusillina and Rissoa are similar in many characters of the shell, radula, head-foot and anatomy. They are regarded as belonging to separate genera primarily because of important differences in their genitalia. Despite these differences the two groups are clearly very closely related as indicated by the large number of shared characters.
Lack of a pallial prostate gland and acquisition of penial prostatic tissue in Rissoa are modifications from what is here assumed to be the more primitive state seen in Pusillina—a pallial prostate gland and a simple penis