The molluscs discussed in this account are from two exposures in the Waihao River: a) the "classical" McCullough's Bridge locality (including the stratotype of the Tahuian ) which has yielded 189 species, 167 of them from the 2 m. thick Tahu Member of early Kaiatan age, the others from beds of late Bortonian to Kaiatan age; b) a recently discovered outcrop of tuffaceous sediments [Kapua Tuffs, also Kaiatan but younger than the beds at a)] from which 46 species are recorded. 26 species are common to the Tahu Member and Kapua Tuffs. Molluscs indicate that the Tahu Member (and probably Kapua Tuffs) was deposited in subtropical waters of normal salinity at depths of at least 150-200 m. Neither fauna shows much similarity to other Arnold Series faunas, largely because of ecological factors. In the systematic section, 206 species-group taxa are recorded and discussed. One new subfamily (of Nuculidae), 20 new genus-group taxa and 78 new species-group taxa are proposed and a further 25 new species are recorded but not named. A new supraspecific classification of the Nuculidae, based primarily on shell structure, is presented and the described New Zealand Cenozoic and Recent species are assigned to the relevant genus-group taxa. Reviews are also given of New Zealand internally costate pectens, the genus Falsicolus Finlay, 1930 (Fasciolariidae) and the New Zealand turrids formerly referred to Marshallena Finlay, 1927 and Marshallaria Finaly & Marwick, 1937. The following synonymies are proposed: Magnatica powelli Laws, 1932 with M. Planispira (Suter, 1917), Tubena Marwick, 1943 with Gegania Jeffreys, 1884, Zexilia tenuilirata and Z. sub-marginata Laws, 1935 with Exilia (Zexilia) waihaoensis Suter, 1917, Eoturris neglecta with E. complicate (Suter, 1917) and Cordieria haasti and C. verrucosa Finlay, 1930 with C. rudis (Hutton, 1885).