Since its discovery in 1938, there has been no agreement as to the zoological nature of the Middle Ordovician jawlike fossil Archeognathus primus from the Dutchtown Formation of Missouri. Early opinions were divided between a vertebrate and a conodont affinity, whereas most recent views have rejected a close relationship with either group. Until recently, the holotype was the only known specimen of this fossil but recent collecting at two Dutchtown localities has added two virtually complete and over 70 fragmentary specimens which provide morphological and chemical evidence on the affinities of Archeognathus. This evidence indicates that Archeognathus primus is a fibrous conodont related to a genus such as Coleodus. Furthermore, we have found no microstructural evidence supporting a vertebrate affinity. Youngquistina mitteni Miller from the Harding Sandstone of Colorado is a second species of Archeognathus, and an unnamed specimen from Alabama is also congeneric, as are probably the fragmentary specimens from localities in the North American Midcontinent and the Siberian Platform that have been consistently referred to Neocoleodus.--Modified journal abstract.