Diagnostic characters
Shell intermediate between that of H. ulvae and that of E. ventrosa as regards degree of swelling of whorls, but narrower than either (apical angle averages 32°); apex rather blunt. The tentacles have a conical area of black pigment close to the tip. The penis is small, rather like that of ulvae in shape but with an angular outline on the right anteriorly.
Other characters
The empty shell is brownish or grey, but living ones seem nearly black. The snout has a dark transverse bar as in ulvae with dark lateral ones and (usually) a median one as well. Up to 3-4 mm high, 2 mm broad; last whorl occupies about two thirds of the shell height, the aperture 35-40%.
H. neglecta inhabits the same kind of habitat as H. ventrosa but prefers more saline water (25%o optimal, but it lives between 10%o and 35%0: Muus, 1963; Cherrill & James, 1985). Its feeding is like that of H. ulvae, optimal with particles 40-80 µm in size, but like that of H. ventrosa in that it cannot take those greater than 120 µm (Lopez & Kofoed, 1980).
The animals have been found in Guernsey, along North Sea coasts, and the west coasts of Scotland and Ireland. It may well be more widespread as it has not been distinguished from the other species until relatively recently recognized in Denmark (Bishop, 1976; Cherrill & James, 1985).