Abstract
Two mountain cyprinid fishes, Onychostoma gerlachi and Scaphiodonichthys acanthopterus, are described from upper Laos.The Vietnamese O.macracanthus is synonymized with S.acanthopterus.As a result the range of the genus Onychostoma is extended westward to Laos from North Vietnam and China and that of the genus Scaphiodonichthys eastward to North and South Vietnam from Burma, Thailand, and Laos.These two genera are closely related to each other and to the Thai-Lao Scaphognathops, the Indo -Burmese Semiplotus, and the West Indian and West Asian Scaphiodon.The distributional patterns of these five genera and related Varicorhinus and Barbus suggest that they originated from northern Asia.Onychostoma has the most northerly distribution and appears most generalized, and Scaphiodonichthys, Scaphognathops, and Semiplotus seem more differentiated.Scaphiodon may represent western offshoots derived from ancestral stock common to Onychostoma.
