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Giant alatoconchid bivalves, a highly distinctive Tethyan fauna, are identified for the first time in the Permian of South China. Alatoconchids range from the lower Kungurian to the uppermost Capitanian in South China. Dense concentrations of the clams signify their gregarious habit.

The study of internal-wave and internal-tide deposits is a new research fi eld on sedimentology during the last ten years. Deep-water traction currents induced by internal waves and internal tides are developed on the modern sea fl oor, which can form not only all kinds of small scale and dispersal deepwater traction currents deposits, but also large-scale sediment wave in km-size scale. In this paper, the concepts and features of internal waves and internal tides in oceanography are introduced, and the characteristics, sedimentary sequences, sedimentary microfacies and sedimentation models of internalwave and internal-tide deposits found and the origin of large scale sediment waves in deep sea bottom are summarized. The relationship between internal-wave and internal-tide deposits and petroleum is discussed, and fi nally internal-wave and internal-tide deposits are shown to be a new potential fi eld for petroleum exploration.

A point-by-point reply is here made to the discussion put forward by G. Shanmugam of our recent paper on internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China. We readily acknowledge that some issues raised by that author are pertinent. However, others are inconsequential, marginal, disputable, or redundant. The available evidence clearly supports our interpretation that the Middle Ordovician Xujiajuan Formation was deposited in a deep-water setting. Furthermore, we uphold that an internal-wave and internal-tide interpretation for these deposits is plausible in view of the occurrence of bidirectional cross-bedding and unidirectional cross-bedding dipping upslope in a deep-water environment. Also, we consider the request to provide conclusive evidence for the existence of a pycnocline in our stratigraphic record as unreasonable-it is self-evident that these data are currently not available, and the absence of proof is not proof of the contrary. Point-by-point reply

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