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Abstract
Coscinodiscus japonicus, Coscinodiscus morsianus and Coscinodiscus solidus are described in order to illustrate three different types of process pattern present in Coscinodiscus species from the Cretaceous and Palaeocene. C. japonicus, representing Group 1, has a single marginal ring of rimoportulae including two macrorimoportulae. C. morsianus (Group 2) has a single marginal ring of rimoportulae including two areas of packed rimoportulae and C. solidus (Group 3) has a single ring of closely spaced small rimoportulae. Groups 2 and 3 apparently disappear during the Eocene whereas species conforming to Group 1 survive and thrive today.
A discussion on the relationships of Groups 1–3 Coscinodiscus species with the closely related genera, Craspedodiscus, Brightwellia and Porodiscus is presented together with some ideas on phylogeny.