New species of the cyathaspidid poraspis (Agnatha: Heterostraci) from the Late Silurian and early Devonian of Northwest Territories, Canada

Four new species of cyathaspidid extend the range of the genus Poraspis back into the Late Silurian and indicate that the Canadian arctic was its center of origin. Poraspis heintzae n. ap., P. cracens n. sp., P. thules n. sp. and P. parmula n. sp. also increase the known geographic range of the genus which had previously been reported only from the District of Mackenzie (NWT, Canada), Spitsbergen, western and eastern Europe. The identification of P. sericea from the upper member of the Peel Sound Formation substantiates correlation of this interval with the crouchi zone of the Anglo-Welsh Borders, and provides another rare example of a species common to the Canadian arctic and European successions.