As will be seen from the list, the shells that have generally been classed under Cardium s. str. are those of the Cardium spatiosum group for which the new genus Maoricardium is proposed. They are huge, thick, veneriform shells with strong, tuberculate ribs, related to the Boreal Cerastoderma and Clinocardium. They are first known from the Duntroonian (Upper Oligocene).

Trachycardium was introduced into New Zealand systematics, on Dall's advice, by Suter (1907) when he described C. waitakiense and the usage has been fairly consistently maintained by subsequent workers. The shells, however, though having the right cardinals cemented together, are very distant from isocardia L., the type of Trachycardium, both in shape and sculpture, particularly in the differentiation of the posterior area; consequently the new genus Hedecardium has been set up. The line is rather commonly represented in New Zealand from the Bortonian (middle Eocene) to the Awamoan (lower Miocene) and the type of sculpture appears to be unique. The group is probably endemic and may have descended from an Upper Cretaceous ancestor which has not yet been discovered. Of overseas species, the Patagonian C. philippii shows some resemblance. It has similar, strong, smooth radials, but the anterior ones are differentiated and the posterior ones slightly tuberculate in a different manner. The Victorian C. pseudomagnum McCoy may also be related.

Much closer relatives to the Trachycardium stock, however, are present in the New Zealand Tertiary. Ovicardium certainly has similar shape and type of ribbing and must have a common origin with Trachycardium and the Australian Regozara. Trachycardium senu lato is very widely and strongly represented in the Indo Pacific region.

Two quite distinct groups were classed by Suter under Nemocardium (as a subgenus of Protocardia); first, the small, thin, Recent and Pliocene pulchellum, since classed in Pratulum Iredale, and second, the much larger species such as alata and sera. Stewart (1930, p. 274) has discussed the Protocardia-Nemocardium-Pratulum groups and given good reasons for generically separating Nemocardium from Protocardia, but he left the relationship of Pratulum to Nemocardium an open question.

The Pratulum line has had a long history in New Zealand, evidence for continuity since the Wangaloan (Danian) being very good. The pre-Pliocene shells do not have the transverse sculpture, including tubercles on the posterior ribs, so well developed as in the Pliocene and Recent pulchellum or the Recent Australian genotype thetidis Hedley. The Chatham Island Tertiary diversum is, perhaps. closer to Nemocardium than to Pratulum, for it has the main part of the disc almost smooth, though it lacks tubercles on the posterior ribs. It is, in fact, difficult to decide which of the two groups should be used for several of the species. The Japanese Pliocene and Recent modestum Adams and Reeve, for instance, has the radials of the disc better defined than those of diversum, and no posterior tubercles, yet geographically it would be considered a Nemocardium, which has been recorded from the China Sea in N. bechei (Ad. and Rve.). In