Light (A, B) and scanning electron (C-I) micrographs of Phryganella nidulus. (A) View of several specimens to illustrate variability in shape and size of the shell. (B, C) Apertural view of two specimens showing general shape and large aperture. (D) Dorsal view. (E) Lateral view. (F, G) Close up view of apertural rim to illustrate its denticulate outline. (H, I) Details of dorsal side of the shell to illustrate its rough surface, covered with large pieces of quartz and diatome frustules.