We shall here give a description of those patterns which are regularly found in Ceylon (for the mechanisms underlying the formation of these patterns see appendix 2). Patterns peculiar to other provinces will be added later. The classification of the Clithon patterns presents con- siderable difficulties. Some are quite distinct and sharply defined so that one is rarely in doubt as to whether a given snail is, for example, a 'purple spiral' or whether it has 'purple-tipped tongues'. Other types are equally characteristic and encountered in every population, such as 'ladders' and 'yellow spirals'. But whereas in some populations these are quite distinct, in others there are varying numbers of intermediates. And some types are connected by so many intergrades that no more than a semi-quantitative classification with arbitrary classes is possible