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A previous geological study of bottom and core samples of fossilized Holocene freshwater thecamoebians, involving the study of thousands of specimens belonging almost entirely to the superfamily Arcellacea, has prompted us to revise some of the rather confused taxonomy of several common genera. The Arcellacea present numerous cases of morphological test intergradation between main phenetic clusters. This has led specialists to propose classifications that recognize almost every rare morphotype as a distinct taxon (species or “variety”). We believe that such intergradation, which is at the basis of the confusion in these genera, is mainly due to the rarity of sexual events. We discuss the rationale and practical criteria for recognizing “species” among such uniparental or almost uniparental organisms. Apart from the problem of intergradation in fossil thecamoebians, taxonomic difficulties also arise from the absence of basic diagnostic characteristics (such as pseudopodia and others). We discuss how the importance of pseudopodial types in thecamoebian microclassification may have been exaggerated. More difficulties stem from the fact that in the past many arcellacean species and varieties have been defined according to the nature of their xenosomes (foreign agglutinated particles). While the shape of idiosomes (test particles secreted by the organism) may be a valid taxonomic characteristic, in most cases the nature of the xenosomes depends on the availability of inorganic particles and not on genome-based selectivity.

The discussion of these problems leads us to define arcellacean species as wide phenetic clusters in which specimens are linked to each other through intergradation. In most cases the intergradational series were selected from large contiguous populations. We have organized accordingly the taxonomy of fourteen species (some of which can themselves be linked to each other by scarcer intermediate specimens) instead of the several dozens of species in which the same organisms were divided by earlier authors. The fourteen species (in five genera) are: Difflugia protaeiformis Lamarck, D. bacillariarum Perty, D. bidens Pénard, D. corona Wallich, D. fragosa Hempel, D. globulus (Ehrenberg), D. oblonga Ehrenberg, D. urceolata Carter, D. tricuspis Carter, Lagenodifflugia vas (Leidy), Pontigulasia compressa (Carter), Heleopera sphagni (Leidy), Centropyxis aculeata (Ehrenberg), and C. constricta (Ehrenberg). These species are redescribed, discussed, and their fossilized forms illustrated. The literature indicates that these species are cosmopolitan and usually form a large percentage of the lacustrine faunae everywhere. Hence the taxonomy presented here is potentially applicable universally.

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