A new family, Calopiidae is described for a new genus and four species, three of them new, from estuarine habitats in Australia. While similar to many rissooideans in having a simple, conical shell, Calopiidae can be distinguished from all other taxa by a unique combinatiom of apomorphic characters, including a simple operculum with calcareous-like opaque-white material on its inner surface. The long cephalic tentacles have complex ciliation, including long, distal compound cilia. The foot has an anterior cleft and the propodium lies dorsally behind the anterior edge of the foot. There is a short metapodial tentacle close to the posterior margin of the foot. The osphradium is longer than the poorly-developed ctenidium, the latter unusual in lacking skeletal rods. The intestine loops around the middle of the style sac, not at its anterior end as in most other caenogastropods, and the rectum forms tight S-shap coil on the penial roof. The testis is composed of a few large, long lobes and the prostate gland is triangular and closed . The large penis has a few large apocrine glands. The female system is unusual in lacking a seminal receptacle and in the oviduct gland being uniform throughout. The spawn consists of short rows of capsules and larvae hatch as swimming veligers. The relationships of the new family are assessed following a cladistic analysis, using 49 informative morphological characters, of similar rissooidean taxa. The strict consensus of the 40 shorlest trees showsCalopiaas the sister of th four taxa in the analysis that are currently included in the Hydrobiidae. A replacement name, Calopia laseroni, is provided forLucidestea perforataLaseron, 1956 a secondary homonym ofLucidestea perforata(Thiele, 1925).
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