 Collected in deposit, the spores of "B. mirabilis" are olive-brown.Viewed with a microscope, the spores are spindle-shaped to roughly elliptical, with smooth, thick walls, and have dimensions of 18–22 by 7–9 µm.Overholts' 1940 publication on the species reported spore dimensions of 20–26 by 8–9 µm.