Dorsally similar; examination of umbilicus (on S. umbilicalis) and columellar tooth (only on Phorcus) advisable. Spire sharper apically on P. lineatus when viewed laterally. Take care with shells under 5mm high, as umbilicus may be open on P. lineatus as well as on S. umbilicalis.
Left: Steromphala umbilicalis, height 11mm, breadth 16.4mm.
Right: Phorcus lineatus, height 12.5mm, breadth 14mm.

Both have worn apex exposing nacre layer and upper end of hollow collumella, and purplish bands radiating from apex.