Diagnosis. Shell nuculoid, obliquely oval, higher than long, monomyarian with anterior adductor muscle scar only. Hinge with subumbonal taxodont teeth and single elongate lateral tooth on the anterior dorsal margin. Ligament mostly opisthodetic, wholly external or in a sunken resilifer.
Remarks. Although living nucinellids are sometimes classified in the family Manzanellidae, which extends back into the Permian (e.g. Coan & Valentich-Scott 2012), Oliver & Taylor (2012) argued that the Nucinellidae and Manzanellidae should be separated on morphological grounds and their conclusions are followed here. Manzanella, the type genus of Manzanellidae, is dimyarian and subcircular in outline, with its taxodont teeth lying posterior to the beak. In contrast, Nucinella and Huxleyia (i.e. the Nucinellidae) are both monomyarian and elliptical, with their teeth positioned anterior to the beak (Oliver & Taylor 2012).