This paper reports the results of investigations on t he ontogeny, variation, dimorphism and structure of carapace in several species of Poloniella Gürich (Kloedenellidae) from Middle Devonian strata of the Holy Cross Mountains (Góry Świętokrzyskie) in Poland. Dizygopleura Ulrich & Bassler is regarded as a synonym of the genus Poloniella Gürich. Two new species: P. brevis and P. diversa have been distinguished; a description is given of the male specimens of two species, heretofore not recorded from Poland (P. tertia Kromrnelbein, P. cingulata Warthin), also of two other species (P. kielanae Pribyl, P. devonica Gürich) previously reported from Poland.