Populations of the Daphnia hyalina-galeata complex from Central, North and South Europe have been compared morphologically. The main purpose of this investigation is delimitation of the critical species Daphnia galeata, based on a thorough examination of the original materials from G. 0. Sars' collection, and taking into account hitherto overlooked or insufficiently appreciated characters. According to their morphology and ecology, Daphnia hyalina, Daphnia galeata and Daphnia cucullata have to be treated taxonomically as separate species. In view of the short and high antennula mound and other morphological characteristics, Daphnia gracilis Hellich and Daphnia hyalina lacustris sensu Lilljeborg belong to the species Daphnia galeata. In the common range of Daphnia galeata and Daphnia cacullata there occurs quite frequently an intermediary form, which according to genetic evidence is a hybrid of these two species. Although apparently this hybrid form is stabilized genetically to a large extent and is capable of identical reproduction, it should not be considered as a species in the usual sense.The present work intends to put the notoriously confused systematics of the Daphnia hyalina-galeata complex on a firm basis. It concludes with a key, which should enable also non-specialists to identify uniquely the three species and the hybrid form in question.

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Fr-ossNrn, D., eNo K. Kneus. 1977. On the variability and taxonomy of Pleuroxus denticulatus Birge (Cladocera: Chydoridae). J. Fish. Res. Board Can. 34: 463476.Comparative morphological investigation of extensive series of Pleuroxus denticulttttts Birge from North America and Europe revealed considerable variability in cerlain characters. These are quantitative (body dimensions), qualitative (structure of postabdomen), and meristic (number of ventro-+audal shell denticles). With regard to the postabdomen, two motphotypes, connected by transitional forms, can be distinguished in both sexes, which, however, are not separated geographically. Neverlheless, there are ceftain field populations (e.g. from Indiana and southem Germany) that are so strongly divergent in certain characters they would satisfy the taxonomic requirements for assigning them to different geographic races. The Nofih American population s are distinguished by having significantly more denticles on the ventro-caudal angle of the valves, the frequency distribution of which satisfied the 75% rule for the separation of subspecies. However, we do not consider the distinctness in this single character of a single west European population sufficient to justify the establishment of two geographic subspecies. Trunk limb morphology, which yields important conclusions conceming the systematic afflnities of Pleuro.rus dentitulutLts, is also described. The species, along with Pleurorus truncutus (O. F. Miiller) and P leu roxu s p ro( ut1, us Birge, form a group of closely related species, characterized by the sperm ducts opening on the lateral surfaces ofthe postabdomen and by the relatively small structural differences between the male and female postabdomen. Finally, differentiation of characters within the truncotus group is analyzed in detail.

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