FOSSIL INVERTEBRATES/Bivalves 369 and rarely with calcified pustules The shell consists of eight imbricating plates, symmetrically arranged in sequence along the centre of the girdle The microstructure of the calcareous shellis aragonitic, and has been likened to the cross-lamellar structure of the conchifera, but the general pattern of the structure differs and is traversed by tubules which house the sensory aesthetes The muscle insertions closely resemble those of the conchifera but they seem to have no substantial myostracum The occurrence of fossil chitons is sporadic but their record is good back to the Early Ordovician A number of probable chitons have been described from the earliest Cambrian (Meisuchun Stage) from Southern China A number of fossils of uncertain affinities may belong to the Mollusca The hyolithids have comparable shell microstructure to some Mollusca but their appendages (helens) and the arrangement of the gut, as occasionally preserved, are quite different Some Machaeridia may be forms intermediate between shelled and non-shelled chitons, as may be the Cambrian Halkeriids and Wiwaxia See Also Fossil Invertebrates: Arthropods; Bivalves; Gastropods; Cephalopods (Other Than Ammonites); Ammonites Further Reading Fretter V and Graham A (1994) British prosobranch mol luscs: their functional anatomy and ecology (Revised Edition), vol 144 London: Ray Society Moore RC (ed.) (1952 1996) Treatise on Invertebrate Paleontology Parts I,K,L,N, Mollusca Vols University of Kansas Press, Lawrence: Geological Society of America Natural History Museum (London) (1996) British Palaeo zoic fossils, edn London: HMSO Natural History Museum (London) (1993) British Meso zoic fossils, edn London: HMSO Natural History Museum (London) (1993) British Caeno zoic fossils Tertiary and Quaternary, edn London: HMSO Pojeta J and Runnegar B (1976) The paleontology of ros troconch mollusks and the early history of the phylum Mollusca Washington, DC: United States Geological Survey Professional Paper No 968 Stanley SM (1970) Relation of shell form to life habits of the Bivalvia (Mollusca) Boulder, Colorado: Geological Society of America Memoir 125 Wenz W and Zilch A (1939 1962) Gastropoda In: Schindewolf OH (ed.) Handbuch der Palaăozoologie Berlin: Borntraeger Wilbur KM and Yonge CM (1964, 1966) Physiology of mollusca Vols New York, London: Academic Press Bivalves E M Harper, University of Cambridge, Cambridge, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The Bivalvia, the second largest class within the phylum Mollusca, is one of the most familiar of all invertebrate taxa Modern representatives, such as mussels, cockles, oysters, and scallops, are well known from excursions to the coast, and in many parts of the world they are important commercial species Their generally excellent fossil record has allowed their evolutionary history to be traced back to the Early Palaeozoic and, for much of this time, they have been important components of many faunas From rather modest beginnings, they have conquered a range of habitats from the deep sea to freshwater, exploited a wide range of life habits (from deep burrowing to swimming), and undergone a near-exponential taxonomic proliferation, a spectacular example of an adaptive radiation Bivalves come in all manner of shapes and sizes, from tiny, thin-shelled commensals that live in association with sea anemones, to giant clams and the extinct rudists and inoceramids which reach(ed) sizes well over m Shell morphology is extremely plastic, but all are modifications of the same basic theme The intimacy of the shell morphology to life habit has been a great benefit in reconstructing the life habits of extinct bivalves, but has also frustrated many attempts to establish the relationships between different groups within the class Bivalves have been proven to be good palaeoenvironmental indicators, but they have only limited use in biostratigraphy Freshwater ‘mussels’ have been used to date fluvial deposits in the Carboniferous Coal Measures of Western Europe, and inoceramids have been used for Late Cretaceous deep marine settings (e.g., in New Zealand) In general, however, species of the