I. Pour species of snails of the genus Lymnœa have been collected from Glamorgan and Monmouthshire in South Wales. A general survey of the area under observation has been given, together with comments on the incidence of infection of these snails by larval trematodes.

II. Over five thousand snails have been dissected with a view to investigating the occurrence, structure, and life-histories of their trematode parasites.

III. Ten different catagaries were found. Seven could be identified and are as follows:—

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The remaining three are new:

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Various structural details have been added to the different stages in the life-cycles of those previoasly known. In C. cambrensis I. the cyst has been found, and in C. limbifera the metacercaria has been associated, for morphological reasons, with an adult echinostome Echinostomum aluid (Nicoll).