Powellisetia unicarinata

Powellisetia unicarinata is a species of marine gastropod mollusc in the family Rissoidae. First described by Powell in 1930 as Notosetia unicarinata, it is endemic to the waters of New Zealand.

Taxonomy
Originally described as Notosetia unicarinata in 1930 by Powell, the species name was revised as Powellisetia unicarinata in 1965 by Winston Ponder.

Description
Powellisetia bilirata has a minute, roughly ovate shell with a single strong spiral ridge. It is coloured dull-white. The species measures 1.19mm, by 0.59mm. The species' shell has a reasonably tall spire and a strongly tilted protoconch, and shell width is the major factor that varies between individuals in the species.

The species is similar in appearance to Powellisetia porcellanoides, but can be distinguished by its angled carnate shoulder.

Distribution
The species is endemic to New Zealand. The holotype dredged by Powell in 1924, from a depth of 9–11 metres at Tryphena Bay on Great Barrier Island. It is found in the north-east of the North Island, as far north as Manawatāwhi / Three Kings Islands,