Type species: H. claviculata (Dall, 1881).
Halonympha was originally described as a subgenus of Cuspidaria to accommodate species with an acute cardinal tooth in the right valve and a posterior 'clavicular' rib in both valves (Dall 1886). Dall (1886) recognized section K (Smith 1885) as being identical, furthermore found a specimen in the U.S.N.M. labelled ‘C. inflata, Porcupine Station 39' (Jeffreys 1881) to be identical to H. claviculata.
The distinctive clavicle or 'buttress' (Dall 1886) is a raised ridge on the inside of the shell that extends along the dorsal edge and, in some specimens, onto the ventral edge of the posterior adductor muscle scar. This rib is much more pronounced in some species than in others.
There are four described species, namely H. claviculata Dall, H. striatella Verrill & Bush, 1898, H. aethiopica Thiele & Jaeckel, 1931 and C. depressor (Jeffreys, 1881), here found to be a Halonympha species. A further, and as yet undescribed, species is present in the collections of the B.M.N.H., labelled ‘C.H. ros Verco' (no. 910. 3. 29. 9594).