Light (A, B) and scanning electron (C-I) micrographs of Heleopera petricola. (A) View of several specimens to illustrate variability in shape and size of the shell. (B) View of encysted specimen. (C-D) Broad lateral view of two specimens showing variability of shell structure. (E) Apertural view to show narrow, closed aperture of encysted specimen. (F) Apertural view showing elliptical aperture with acute commissures. (G) Lateral view of aperture. (H) Close up view of aperture showing the bordering thin collar of organic cement. (I) Portion of shell to show arrangement of particles in anterior part of the shell.