Abstract
Species of the testate rhizopod genera Arcella and Cyclopyxis are found predominantly in the benthos of shallow ponds, forest pools, marshes and other freshwater habitats. Their taxonomy is based on the morphology of their tests — the shell-like structure that houses the living single-celled amoeboid protoplast. This paper describes a new species of Cyclopyxis discovered in two coniferous forest bogs, and a new species of Arcella from a Typha-dominated marsh on the north shore of Lake Ontario, Canada. Cyclopyxis acmodonta Nicholls n. sp. differs from its closest relative C. stellata (Wailes) Deflandre 1929 in its much smaller size and its irregularly shaped pseudostomal aperture, which is embellished with numerous, tiny, sharp-pointed toothlike quartz granules around the interior margin. Although the saucer-like indentations over the dorsal surface of the test of A. formosa Nicholls, Meisterfeld & Török n. sp. resemble similar structures found in several other Arcella species, the large size of A. formosa (165-235 mm in diameter) and its low profile (low height-to-diameter ratio) are the main features distinguishing this new species from other Arcella species.
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