First records of Arctodiaptomus byzantinus (Copepoda: Calanoida) from the East European Plain (Russia) and A. similis from Mongolia

The calanoid copepod Arctodiaptomus byzantinus Mann, 1940 (Copepoda: Calanoida: Diaptomidae) is recorded for the first time from the East European Plain (Middle Volga Region, Russia), and A. similis (Baird, 1859) is found in Darkhat Hollow (Mongolia). Brief redescriptions and illustrations of both species are provided. Additionally, certain morphological characters are clarified, particularly those of the leg 5, antennulae and caudal rami.