Full detailed descriptions of the two species of Obtusoecia, one of two planktonic halocyprid ostracod genera that are bipolar, demonstrate that the taxonomic separation of these two forms formerly considered to be conspecific, is valid. The segregation of the genus from Porroecia is also validated. The value of characters of limbs other than the first and second antennae particularly in defining halocyprid genera is emphasised. Zoogeographical distributions of the two species based on comprehensive compilations of both published and unpublished data show that O. obtusata is confined to the North Atlantic, whereas O. antarctica has an Antarctic circumpolar distribution. Detailed bathymetric profiles show that O. obtusata is a shallow mesopelagic species that is overwhelmingly dominant at depths of 50-200 m in subpolar seas, and shows limited ability to submerge at lower depths, so that it is restricted to seas that have a marked seasonal cycle of turn-over and stratification. It is postulated that the bathymetric distributions of the two species are similar, also that O. antarctica is more likely to be ancestral to O. obtusata than vice versa.
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