BRIEF DESCRIPTION: The description that follows is based upon the treatment of this species by Bas (1982).

The cap of A. hyperborea is up to 60 mm wide, convex to plano-convex, probably originally umbonate, dull to somewhat shiny, thin-fleshed, with a rather strong and dense sulcate margin (20 - 50% of the radius).  The cap is white in the center.  The volva is present as small, subconical white warts or felted patches in the center of the cap.

The gills are free, crowded, probably rather narrow, probably with a minutely flocculose edge, and white.  Short gills are present.

The stem is relatively short, up to 40 mm long, moderately thick, white, without a ring, slightly fibrillose, with a narrowly clavate to subbulbous base. When dried, the base is 75% wider than the stem.  Volval warts or patches are small, vague, white, felted subflocculose, and present on the lower quarter of the stem.  In Bas' drawing, he shows the volva rather low on the stem.

The spores measure 11.2 - 13.3 (-13.8) × 9.4 - 10.8 (-11.3) µm and are inamyloid and subglobose to broadly ellipsoid, rarely ellipsoid, sometimes broadened subapically so that they appear egg-shaped.  Clamps are absent at bases of basidia.

It was originally described by Karsten in Russian Lapland.  Earlier Karsten had found what he believed was the same species in the bank of the river Tuloma in Russian Lapland among grasses in the sandy bank of the Tuloma River.