Left: Steromphala umbilicalis, breadth12.3mm with 23 bands (at perimeter, including lip). Umbilicus circular.
Right: Steromphala cineraria, breadth 12.5mm with 48 bands. Umbilicus oval.
FURTHER DATA BELOW

In a sample of 71 shells of S. umbilicalis and S. cineraria, collected from sites between north coast Scotland and south coast England, the average number of bands within each width category on S. cineraria was approximately double the average on S. umbilcalis, with no overlap in the ranges. Examples:

Width 14mm to 15.9mm
S. umbilicalis mean 22 bands (range 18 –31 bands).
S. cineraria mean 52 bands (range 35 –68 bands).

Width 12mm to 12.9mm
S. umbilicalis mean 17 bands (range 13 –23 bands).
S. cineraria mean 41 bands (range 34 –49 bands).

Width 5mm to 8.9mm
S. umbilicalis mean 10 bands (range 7 –13 bands).
S. cineraria mean 25 bands (range 19 –29 bands).

Bands were counted around the periphery of the body whorl, including part of the outer lip to complete the circuit.