VARIATION IN SNAIL SPECIES WIDESPREAD ON PORTO-SANTO, MADEIRAN ARCHIPELAGO

Shell variation has been studied in two widespread land snails from Po rto Santo, Madeiran archipelago: the helicid species or species comple x Heterostoma paupercula (Lowe) and the clausiliid species Boettgeria lowei Albers. Samples were collected from the mainland and the offshor e islets from sea level to the highest altitude. Shell size, colour an d presence or absence of peristome teeth were scored in H. paupercula and some individuals were dissected to examine genitalia. Shell size a nd rib number were scored in B. lowei. In B. lowei size declines and r ib number increases with altitude. In H. paupercula samples from sandy locations have a larger shell size and a higher frequency of pale col our than those from non-sandy locations. On average, sandy sites have a lower altitude than non-sandy ones, however, so that these two varia bles are confounded. The survey shows that shell characters of a kind used in taxonomy are subject to variation associated with ecology; thi s must be borne in mind when assessing present-day taxa and morphologi cal change through the fossil sequence. The data for H. paupercula sup port the conclusion that it is a single species variable in shell char acters and polymorphic for hemiphally.