In North Vietnam, the Hua Tat, Pa Khom and Lang Son Formations yield abundant Lower Triassic bivalves. The occurrence of Eumorphotis sp. cf. E. multiformis, represents the first fossil reported from the Hua Tat Formation, while Claraia aurita, C. concentrica, and C. intermedia multistriata are found in the upper Lang Son Formation. The Pa Khom Formation contains C. intermedia multistriata, Claraia sp., and Lopha sp. The occurrence of C. intermedia multistriata indicates that the Pa Khom Formation and the upper Lang Son Formation are Olenekian in age. These Claraia spp. are found in wave-dominated nearshore deposits characterized by hummocky cross stratification and wave-ripple laminations, although Claraia in other areas is generally found in deep-sea mudstone.