1. Darwin, C. On the Origin of Species (Murray, London, 1859).
2. Gheeraerts, M. De Warachtighe Fabulen der Dieren (Pieter de Clerck, Brugge, 1567).
3. Cockerell, T. D. A. Nature 108, 496–497 (1921).
4. Rees, W. J. Proc. Malac. Soc. Lond. 36, 269–282 (1965).
5. Gray, J. E. Zool. J. 1, 61–62 (1824).