Part of a thesis accepted for the degree of M . Sc . in the University of London .)
1 . INTRODUCTION
A considerable amount of work has been done linking epiphytic freshwater algae with various substrata, (e .g . FRITSCH (1929), BUT-CHER (1932a) and (1932b), GODWARD (1937), and LUND (1942) ; but molluscan shells have received only brief mention . Thus WEST (1904), HEERING (1914), DELF (1915) and STRATTON (1946) comment on the presence of algae in such situations but dismiss the matter in a sentence .
The freshwater gastropods, of which there are thirty-six species, live mainly in the surface two metres, while the Lamellibranchs, twenty-six species, are normally to be found either in or on the bed of the water, be it pond or river . The range in size, shape, speed of movement and length of life is considerable and although the chemical composition of their shells varies but little, the surface characters, for example smoothness, display some variety . With these differences in mind the green algae living epiphytically on the shells of fifteen species of snails and about three species of bivalves were investigated .
2. Methods
The molluscs were collected from a wide range of habitats in England and Wales including small stagnant ponds, tarns, lakes, a reservoir, a small stream and rivers .