Rissoa membranacea (centre & right) and Rissoa guernii (left).
See nature22.com/estran22/estran.html
Rissoa guernii lives on Zostera and other weeds, and it may be confused with ribbed forms of R. membranacea on the south coast of England, Bretagne and further south. Distinguishing features are the aperture’s smoothly rounded adapical angle (1) and fine spiral ridges (2) which are most distinctly visible where they cross the grooves between the large costal ribs. In this view, the large ribs are vertical (orthocline) or slightly tilted with the adapical end to the right (opisthocline) on R. guerinii, but on R. membranacea the ribs are tilted with the adapical end to the left (prosocline). Bretagne, France. © Marc Cochu.