Sigmoilina sigmoidea, from Cushman (1917) (2a, lateral view; 2b, aperture view; 3, axial cross section). Modified from image available on the World Register of Marine Species (WoRMS Editorial Board 2017).  
  Part of: Egloff W, Agosti D, Kishor P, Patterson D, Miller J (2017) Copyright and the Use of Images as Biodiversity Data. Research Ideas and Outcomes 3: e12502.