The minimum information required for a complete diagnosis of coiled shells is discussed with the help of an advanced mathematical model of a shell. It is concluded that, in addition to a good, coloured, picture plus the main dimensions of the shell, information is required about either the number of whorls, or, preferably, the dimension of the apical whorl. In view of a planned conchometrical analysis, a few investigations have been performed with regard to the accuracy with which whorl expansion rate A and height over width ratio Wn/Dn of the whorls can be measured.