Some Lower Cretaceous nonmarine bivalves from fluvio-lacustrine deposits bearing dinosaur fossils in Mongolia and northeast China

One species of Unio and three species of Sphaerium, the typical nonmarine bivalves in East Asia, from the Lower Cretaceous fluvio-lacustrine deposits of Choyr Basin of southeastern Mongolia and the Beipiao area of Jehol in northeastern China, are described. The assemblage of Unio longus (Zhu), Sphaerium chientaoense Suzuki, and S. coreanicum (Kobayashi and Suzuki) from the Khuren Dukh Formation of the Choyr Basin of southeastern Mongolia can be compared with the Nakamuranaia-Margaritifera (Menyinaia)-Neomiodonoides assemblage in China. The four nonmarine bivalve species described herein are found in numerous formations in East Asia suggesting Valanginian to Albian age.