Genus Nucinella Wood, 1851
Type species. Pleurodon ovalis Wood, 1840
Remarks. Nucinella has sometimes been placed in the family Manzanellidae Chronic, 1952 (e.g. Amano et al. 2007; Kiel et al. 2008a). However, the Permian genus Manzanella Girty, 1909, is longer than high, with roughly equidistantly positioned beaks. Furthermore, it is dimyarian, with anterior and posterior adductor muscle scars roughly equal in size (Chronic 1952). This contrasts with the genus Nucinella Wood, 1851, which is monomyarian and has a very short posterior shell margin (e.g. Allen & Sanders 1969). A dimyarian condition was previously postulated for the supposed sister genus Huxleyia Adams, 1860 (e.g. Habe 1958; pl. 9, fig. 16; La Perna 2004, p. 571). However, investigation of Huxleyia habooba Oliver & Taylor, 2012, and Huxleyia sulcata Adams, 1860, revealed no sign of a posterior adductor muscle, which supports separation of Nucinella and Huxleyia into a separate family Nucinellidae (Oliver & Taylor 2012), with Manzanellidae restricted to the genus Manzanella.