Terrestrial gastropods have been widely used to investigate palaeoclimates by measuring the stable isotope signals of δ£18 Oxygen and δ£13 Carbon bound up within the shell carbonate. These signals can act as proxies for the local precipitation and/or relative humidity and the vegetation from which the snail obtained its moisture and food, and thus can be used to infer the wider climate at the time of shell formation. Results will be presented to illustrate that all land snail shells are not equal£ some shells demonstrate wide within-shell variation, and within-species variation is just as great. It will be shown that inadequate homogenization of the shell or measurement of only part of a shell could lead to highly erroneous conclusions. Even where homogenization is possible, the large intraspecific variation found in Canarian land snail shells requires that researchers ensure a sufficiently large sample size that takes into account the natural variation present.