Note
Existing keys to Brissopsis rely partly on the morphology of the labrum, an unpaired post-oral plate. As documented here, the unreliability of labral morphology has frequently resulted in misidentification of B. atlantica Mortensen, 1907 as B. elongata Mortensen, 1907. A revised key to western Atlantic species, using the number and shapes of plates below the periproct as substitutes for labral morphology, is provided.