Standardisation method for counting the tubercles on the shell surface. AB Maximum diameter CD Maximum height EF height of last whorl HG height of second whorl IC height of third whorl; peristoma angle, measured as the angle between the peristomal plane and the horizontal axis of the shell JK width of the last whorl measured along the maximum diameter (JK/2)2 square on the lower surface of the last whorl delimiting the area for the count of tubercles. The centre of the square corresponds to the mid point between J and K and the length of its sides is JK/2. This method allows to compare different specimens and taxa as the area of the square depends on the shell’s overall dimensions and is always proportional (the tubercles that partially traverse the contour of the square were included in the count of tubercles).