Left: Gibbula umbilicalis, breadth 6.5mm with 11 bands (at perimeter, inc. outer lip), August 1969, NW coast Scotland.
Right: Gibbula cineraria, breadth 6.5mm with 26 bands. Aug. 2013. Menai Strait, Wales.
FURTHER DATA BELOW

In a sample of 71 shells of S. umbilicalis and G. 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.