This study is a contribution to the Swiss National Foundation (project no. 200021-113616 to H.B.). W. Blanckenhorn (Zoological Museum, University of Zürich) is thanked for the loan of gastropod material. Technical assistance of M. Specht and F. Hilti (Anthropological Institute, University of Zürich) has been deeply appreciated. Discussions with colleagues, especially S. Urdy, also contributed to improve the proposed method.