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The paper is a taxonomic study of the limnetic Daphnia populations of the English Lake District, the populations upon which the most popular taxonomic scheme is based (Johnson, 1952).

Johnson treats these populations as three varieties of the species D. hyalina, namely D. hyalina s. str., D.h. var. galeata and D.h. var. lacustris.

Careful examination of the populations of Esthwaite reveals that what has previously been regarded as a population of a single variety of D. hyalina. includes three distinct forms. Experimental work confirms that these are genetically distinct. One of these forms corresponds to D. hyalina var. galeata, whilst the other two are designated the “toothed” and “round-headed” forms.

The first part of the paper is concerned with the taxonomic status of var. galeata. Brooks (1957) and some later workers treat this as a separate species (D. galeata) on morphological grounds. This is fully supported by the present work. Differences between var. galeata and co-existing forms in incidence of sexual reproduction, in vertical distribution and in seasonal abundance and average brood-size provide further support for the separation of D. galeata.

The second part of the paper is concerned with D. hyalina s.str. which, on the basis of morphology, ecology and reproductive behaviour, is shown to be at least as distinct from other members of the complex as is D. galeata. It is argued that other forms should therefore to be removed from D. hyalina. The taxonomic status and affinities of the remaining “toothed” and “round-headed” forms are discussed.