« Of a squarish form, with regular ribs of medium size and number. The margin is generally crenuled, but the variety named Crassina Scotica is not so. The name A. Banmoniensis is lost, on account of the priority of the specific name sulcata. In this […] species the ribs are frequently nearly obsolete towards the ventral margins and back. » – G. B. Sowerby: Thesaurus conchyliorum vol. II, London 1855, via BHL.
In fact, the crenulations of the margin often appear together with its thickening, and could be related to the age of the animal (MBSBI).

Above: a juvenile from infralittoral, Giglio island, Toscana, W. Italy. 2,8mm. The increasing of the commarginal beads is less obvious than in fusca, their widths becoming rapidly constant; also these beads are much more marked and they never vanish near the margins as in the latter.