Sigmoilina sigmoidea, from Brady (1884)¬†(1a, 2, lateral view; 1b, 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. https://doi.org/10.3897/rio.3.e12502