EARLY JURASSIC BIVALVE FOSSILS FROM THE XIWAN COAL MINE, GUANGXI

This paper describes the Early Jurassic non-marine bivalve fossils collected from the Daling Fromation at Xiwan, Zhongshan county, Guangxi. These bivalves were found in a bed of black limestone, about 62 m thick. The fossils include one genus and five species as listed below: Qiyangia loxos, Q. fortis, Q. lilingensis, Q. cuneata and Q. cf. loxos. The microstructure of Qiyangia shells has also been preliminarily studied. The shell structure of Qiyangia is described below: test about 14 mm thick: lenticular nacre well developed, about 10 mm thick; it consists of nacre tablets regularly arranged, and bended upwards, paralle to the growth line, downwards at an angle of 40-45 to the sheet nacre （about 3 mm thick）. There are more sheets of organic matrix in the sheet nacre, but conchiolin layers within the nacre are not seen. This genus differs from Margaritifera and Unio in having thicker lenticular nacre, more bens, thicker sheet nacre and no conchiolin sheet. Judging by the macrostructure and microstructure of Qiyangia. the authors consider that it is right to set up a genus and it is temporarily ascribed to Unionidae.