In New Zealand, as in most places, the generic name Cardium has always been used in a very wide sense, and poorly preserved fossils are likely to require its continued use. For those species however, of which we know adequate details of sculpture and hinge, more restricted generic grouping must be used if we are to obtain from them the stratigraphic and distributional evidence that they offer. The now generally accepted type of Cardium, the Recent Indo-Pacific C. costatum Linné (designated by Children, 1823) has strong, high, sharp ribs. No relatives at all close are known from New Zealand, so the use of the genus here can only be in a family sense.

Of the two Upper Cretaceous species figured by Woods (1917, plate 18, figs. 3, 4, and 5) as Cardium spp., the original of fig. 3 must still remain thus vaguely designated, little further evidence having been found. The specimens of figs. 4 and 5, however, belong to Ethmocardium. The other Cretaceous species are all related to Lahilia and have been classed in a separate sub-family Lahilliinae (Fin. and Marw., 1937).

In her most useful summary of the Cardiidae, Dr. Myra Keen (1937, p. 13) excluded Lahillia from the family, suggesting its relationship to Thetironia Stoliczka. Much uncertainty has prevailed as to the family affinities of Thetironia, the general tendency being to put it in the Veneridae. This was done mostly without a true knowledge of the hinge, as is shown by Dall's statement (1903, p. 1283) that it had three cardinal teeth. Woods (1907, p. 165) has since clearly shown by his description and figures of the genotype that the cardinal teeth are on the Cardiid pattern, even to the cycloid arrangement.

The Lahillia hinge is so like the Cardiid one (lacking only the anterior lateral tooth) that relationship is certainly closer to the Cardiidae than to any other family that has yet been proposed. Whether Lahillia should remain in the Cardiidae or not is merely a question of how comprehensive a family should be. The subfamily Lahilliinae was proposed so as to draw attention to the differences from the Cardiidae and at the same time not lose sight of the affinities to it. The same effect is got by recognising the family Lahilliidae as a member of the super-family Cardiacea. Although Cardiids are so numerous, and so varied in many of their characters, their hinges are very conservative. Consequently the complete absence of an anterior lateral in Lahillia may be taken as of greater importance than usual for such a character. This, together with the tendency to sinuation in the pallial line, and the lack of sculpture or of radial structure in the shell (including absence of marginal crenulations) induces the writer, following Frizzel's (1936) treatment of the Veneridae, to elevate the Lahilliinae to family rank as the Lahilliidae of the super-family Cardiacea.

The Cardiids of New Zealand fall into six distinct groups, easily distinguished, and without intergrading or intermediate members.