The basal unit of the Jefferson formation at the head of Spring Mountain Canyon, Lemhi County, Idaho contains the following fishes: a heterostracan, Psephaspis idahoensis n.sp., which is shown to belong to the Pteraspididae rather than the Drepanaspididae; the arthrodire, Holonema haiti n.sp., and undetermined arthrodires; an antiarch referred to the Astrolepididae indet.; and a spine possibly belonging to a holcephalian. The age is considered to be Middle Devonian, and the depositional environment possibly estuarine Includes bibliographical references (p. 288) The basal unit of the Jefferson formation at the head of Spring Mountain Canyon, Lemhi County, Idaho contains the following fishes: a heterostracan, Psephaspis idahoensis n.sp., which is shown to belong to the Pteraspididae rather than the Drepanaspididae; the arthrodire, Holonema haiti n.sp., and undetermined arthrodires; an antiarch referred to the Astrolepididae indet.; and a spine possibly belonging to a holcephalian. The age is considered to be Middle Devonian, and the depositional environment possibly estuarine Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-)