Abstract: Abundant bivalve fossils were first collected from a volcanic rock series（Yaiba Formation）on the southern margin of the Lhasa massif.They include 14 species and indeterminate species.This indicates that volcanism in the Yaiba Formation took place in the Middle Jurassic Bajocian Stage and proves that the early Middle Jurassic was an important stage in the formation of the Neo Tethys ocean basin.The marine bivalve fauna discovered in the Lhasa massif contains typical elements of the East and West Tethys biogeographic provinces.The authors hold that the “Bangong Co Nujiang Sea”did not constitute a geographic barrier to the ocean basin nature of the area between the Lhasa massif and the Qiangtang massif for the interchange of faunas.