Abstract

Neither of the 2 new species from the Upper Silurian of Cornwallis Island, Northwest Territories, Canada, is represented by a complete specimen, but the disarticulated material of one is silicified. The morphology of the holaspid exoskeleton is revealed in detail and the development from the late meraspid onward. The second species lacks marginal spines on larger pygidia. Both species are like H. ptyonurus from the Upper Silurian of New York State, the original material of which is redescribed. The 3 species belong to a group of Hemiarges species differing in some respects from the type and like species. The distinctive characters of the exoskeleton are combined in so many different ways in Hemiarges species, however, that subdivisions of the genus are not now practicable.

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