Australian Middle Cambrian molluscs and their bearing on early molluscan evolution

Twenty-eight species of fifteen genera of Middle Cambrian molluscs are described from tiny phosphatic moulds or silica replicas of the shells. The molluscs were etched from limestones at two sites: one in the earliest Middle Cambrian Coonigan Formation of the Mootwingee area, 130 km northeast of Broken Hill, New South Wales; and another in the middle Middle Cambrian Currant Bush Limestone of the Thorntonia area, 150 km northwest of Mt Isa, Queensland. These unusually diverse collections show that many different kinds of molluscs lived in the tropical Australian seas of the Middle Cambrian and provide new information on the way the molluscan classes Cephalopoda, Gastropods, Rostroconchia, and Pelecypoda evolved. In other sections, we discuss the problems of classifying and naming Cambrian molluscs; define a number of terms that can be used to describe shell form (including a new adjective, gyrogastric); reclassify the Class Monoplacophora after incorporating the helcionellacean and bellerophontacean 'gastropods'; and outline the early record and history of the Mollusca. New taxa are: the Families Scenellidae (nom. transl, ex Scenellinae Wenz 1938) and Yochelcionellidae; the genera Eotebenna (Yochelcionellidae), Mellopegma (Procarinariidae), and Protowenella (Multifariidae); and the species Helcionella terraustralis, Latouchella accordionata, L. merino, L. penecyrano, Yochelcionella daleki, Y. ostentata, Eotebenna pontifex, E. papilio, Mellopegma georginensis, Stenotheca tepee, S. pojetai, Protowenella flemingi, Pelagiella deltoides, P. corinthiana, and Myona? queenslandica. B. Runnegar, Department of Geology, University of New England, Armidale, Australia 2351 and P. A. Jell, Department of Geology and Mineralogy, University of Queensland, St Lucia, Australia 4067; received 22 December 1975.