Ilyocypris Brady & Norman, 1889 is a globally distributed freshwater ostracod genus, with 27 Recent species described so far. In the present paper we describe I. hanguk, new species, collected from a stream in South Korea. This species is very closely related to I. dentifera Sars, 1903 and I. angulata Sars, 1903, both originally described from China and also reported from Korea and Japan. Due to their limited descriptions, the two species were often synonymized in the past, and I. angulata was considered a parthenogenetic form of I. dentifera. In order to re-examine morphological characteristics of I. dentifera and I. angulata, as well as to compare them with the new Korean species, we have studied the type material deposited in the Zoological Museum in Oslo. This revealed that I. dentifera Sars, 1903 and I. angulata Sars, 1903 are valid species, with clear morphological differences. We have also discovered males of I. angulata in the type series. In addition, another form, which we identify as I. cf. angulata, is found within the type series of both species. Ilyocypris cf. angulata has an intermediate carapace form between I. angulata and I. dentifera. Dissection of its soft parts, however, indicates unambiguous difference from I. dentifera, and a high similarity with I. angulata. It was left in the open nomenclature because many species of the genus Ilyocypris are still very poorly known and characters of the hemipenis that may define this form as a clear species are missing from many species descriptions. Ilyocypris hanguk can be easily distinguished from these two species by the morphology of the hemipenis, and other details of the soft parts. Although a widely distributed species, Ilyocypris bradyi Sars, 1890, is recorded from South Korea for the first time. This is also the first record of the bisexual population, and we describe males in detail. In the present paper we also provide a cladistic analysis of the genus Ilyocypris based on 20 morphological characters and 21 species.

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