A new species of the cythaspid Poraspis is described from the Emsian (Early Devonian) Lippincott Member of the Lost Burro Formation in Death Valley, California. Poraspis thomasi, sp. nov., extends the geographic range of the genus, which had previously been reported from western Europe, Spitsbergen, the Canadian Arctic Archipelago, and District of Mackenzie. In addition, the temporal range is extended as the previously described species ranged through the Late Silurian into the Pridoli. This species includes articulated specimens showing that the oral area consisted of a single rectangular oral plate, in contrast with the view that in cyathaspids the area consisted of an arrangement of numerous finger-like plates. Also, one ventral shield shows growth lines indicating that growth of the shield occurred in a posterior direction. An additional poraspid, Lechriaspis patula, gen. et sp. nov., is described from a single specimen from the Grassy Flat Member of the Water Canyon Formation, northern Utah, that shows evidence of puncture wounds indicating predation by a eurypterid. These new heterostracans represent the first true cyathaspids to be described from the Devonian of the western United States.