Barnesoceras is a genus of moderately large, Lower Ordovician ellesmoerceratids with a simple, but strongly shell that expands in the vertical dimension more rapidly than in the horizontal, resulting in its being laterally compressed. The aperture is simple, unconstricted, and straight sided from in front. Growth lines show a broad crest on either side that swings forward above the ventral . Septa are close spaced and have a shallow curvature. The body, or living, chamber takes up much of the shell. The siphuncle is ventral, close to the longitudinally concave side.