Opisthobranch gastropods with thin shell, less than 1 cm high, with an umbilicus and a groove between whorls or an involute spire. Rhinophores simple, triangular flaps in front corners of head-shield. Gizzard plates and parapodia absent. Radula with formula 1.1.1 or 1.1.1.1.1 consisting of roughly rectangular median teeth, strong, angled lateral teeth with a hacksawlike inner edge, and, when present, thin, pointed marginal teeth. Lateral and marginal teeth curving forward and inward over median teeth.
Description
Shell: External, thin, ranging in shape from cylindrical to globose. Surface sculpture in most species consisting only of faint growth lines; in a few cases there are other sculptural details, but these are never very prominent. The shell can be transparent, white or whitish in colour, sometimes with light brown bands or uniformly light brown. Spire ranging in shape from acuminate to involute, but if involute always with a small, apical opening down to the protoconch. The suture between whorls forms a groove, except in very concave and involute spires. Shoulders generally rounded, rarely angled. Number of teleoconch whorls seldom more than 3.5 in adult specimens. The shell aperture usually seems of about the same size as the body whorl when the shell is seen from the apertural side; a few species with involute spires have larger apertures. Columella an open spiral, forming a distinct, although sometimes narrow, umbilicus. Protoconch 0.25-0.65 mm in diameter with either less than one whorl or 1.3-2.25 whorls.
External anatomy: Foot broad; forked into two triangular flaps posteriorly or with a more con-ventional, blunt end. Parapodia absent. Head shield forming two triangular flaps on its front corners. In many species eyes are clearly visible from above, embedded in the head shield, which is (semi-)transparent.
Mantle cavity: Gill small and narrow, not hanging far down from mantle roof. Kidney located adjacent to gill, to the left of and behind it. Along the posterior left wall of the pallial cavity, the intestine can clearly be seen running back to the posterior end of the cavity. Here the rectum bends towards the right side of the animal with the anus situated about midway from the left wall. Genital organs located far forward on the floor, just behind the neck. No pallial caecum.
Radula: Formula 1.1.1 or 1.1.1.1.1. Can be asymmetrical, that is, the teeth can show stronger development on one side than on the other. Broad, roughly rectangular, median tooth with denticles on the free edge, which has two backward-curving lobes. Each lateral tooth has a strong base shaped approximately like a triangle with two short legs; the cusp, which projects from the top of the triangle, curves away sharply from the plane of the latter to an angle of 90°, after which it straightens and continues to a rather blunt end. The inner curve and edge of the lateral tooth have a very fine, sharp dentition like a hacksaw. Each marginal tooth, when present, is about 2/3 the length of the laterals, but very thin, with a weaker base and no dentition along the edge. The marginal tooth ends in a sharp, crooked point. Laterals and marginals bending forward and inward over central teeth, those from the right side overlying those from the left.

Resemblance to other genera When soft parts are disregarded, some Diaphana species can be mistaken for members of Retusidae, and indeed often have been so. This is especially true for species with a slender shell, which resemble some of the well known species of Retusa Brown, 1827, while Diaphana species with a more pentagonal shell shape can be confused with some species of Pyrunculus Pilsbry, 1895. Within the Diaphanidae there is a great.deal of resemblance between the shells of Colpodaspis M. Sars, 1870 and the more glo¬bose Diaphana species, as well as between some Toledonia Dall, 1902 and Diaphana species. Retusa species usually don't have a true umbilicus, Pyrunculus and Colpodaspis have protoconchs, apical regions and/or shoulders different from those found in Diaphana. Toledonia shells are usually more acuminate than those of Diaphana species. In all cases it may, however, be necessary to resort to the anatomical differences. The lateral teeth of the Diaphana radula are diagnostic.

Widely distributed in Arctic and Antarctic areas and northern temperate regions from shallow water to deep sea. Distribution in southern temperate regions less well known: Widely distributed in New Zealand and southern Australia and occurring commonly in southernmost South America. Subtropical occurrence at least in the Mediterranean and Baja California. Tropical occurrence only noted on Cape Verde Islands and in Mauretania. In the deep sea the deepest known occurrences are about 4500 m in the northern and central Atlantic.

Shell thin and fragile, capacious or subglobose, umbilicated, the spire either projecting flat, or sunken in a narrow apical umbilicus. Aperture narrowed above, rounded below, the lip sinuous; columella not thickened, long and rather straight, neither folded nor truncated, its edge a little reflected above.
TYPE. Diaphana minuta Brown.
DISTRIBUTION. Northern United States, Europe.