Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 18. 1996. (Budapest, 1996)

VÖRÖS (1991), who convincingly demonstrated that this peculiar type of rocks was deposited on slopes and in fissures of submarine highs. Relatively thick, stratigraphically more or less continuous sequences of variegated crinoidal, spiculitic and cherty biomicrites and biosparites as well as those of red, nodular ammonitic wackestones and mudstones (ammonitico rosso limestones and marls) sometimes interfingering with the former lithology characterize the basins between submarine highs. Frequent occurrence of allodapic beds in the former type of sequences indicate that their depositional areas were in proximity of seamounts from where the redeposited particles originated. Fig. 2. Lithological column and zonal stratigraphy of the bivalve localities indicated in Fig. 1. A: red, Fe-Mn-oxyde rich limestones; B: Hierlatz limestones; C: variegated crinoidal and spiculitic or cherty limestones; D: Ammonitico rosso limestones. (Drawn after VÖRÖS 1995) MATERIAL AND PREVIOUS STUDIES Nearly 500 specimens, originated from 12 localities, form the base of this study. About five sixth of the material has been collected by the staff of the Hungarian Geological Institute. The fauna represents the whole Pliensbachian. Zone-by-zone changes both in species and specimen numbers correspond to the spatial and temporal distribution of favourable environments, as reflected by the changing lithologies. The Spinatum zone represented exclusively by basinal sediments has proved to be the poorest in bivalves. A considerable range of modes of preservation is displayed by the material. Valves originated from red limestones rich in Fe-Mn-oxydes as well as those from Hierlatz limestones are usually preserved with original or recrystallized calcitic shell. Shells are often coated with, or replaced by ferro-manganese-oxydes in the former lithology and they bear traces of bioerosion. The hard rock matrix makes the study of internal features difficult. Among specimens yielded by basinal sediments internal moulds prevail. Features of the shell exterior have been usually compressed onto the internal molds, indicating an early dissolution of shell material.