The lycophyte Northern Quillwort (Isoetes septentrionalis D. F. Brunton, sp. nov.) is the northern component of the Riverbank Quillwort (I. riparia) complex. It is locally abundant in southeastern Ontario and southwestern Quebec and is also widely distributed in the northeastern United States. In Canada, it is largely confined to river and lake shores along post-glacial drainage outlets within the limits of the post-glacial Champlain Sea. It is frequently found in association with other

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... especially Dodge's Quillwort (I. × dodgei A. A. Eaton), its sterile hybrid with Spiny-spored Quillwort (I. echinospora Durieu). The Canadian population of I. septentrionalis appears to be stable. Although first proposed at species level over a century ago, the nomenclature of this taxon has remained unresolved. It is clarified in the present study, in which I. canadensis var. robbinsii is lectotypified.