A new benthic foraminiferal genus and species of Heleninidae was discovered in estuarine sediments of the Sihchong River, western Hengchun Peninsula, Southern Taiwan. Similar to the resembling genus Helenina, Cyclohelenina gen. nov. displays distinct sutural incisions on both spiral and umbilical sides. However, it exhibits relatively coarser perforation of the wall, more roundness along the equatorial outline, and a wider range in the number of chambers (7–10) in the last whorl compared to other Heleninidae genera. This new genus can provide a suitable taxonomical unit for the assignments of several undetermined species that were previously classified to but distinguishable from the genera “Pseudoeponides” and Helenina.

In this research, we examined and measured 58 syntypes and one ideotype (from the Kangkou River on the east flank of Hengchun Peninsula) of Cyclohelenina formosaensis sp. nov., the type species of the new genus. Under the concept of functional morphology, the hispid ornamentation found to be present around every opening on the test of C. formosaensis suggests that it may be an herbivore. Observations of yellowish-green stains within fresh tests (evidence for chlorophyll from their food source) are consistent with this inference. The test with low-trochospiral shape and sutural incisions on both sides indicated that C. formosaensis may prefer hyposaline habitats with sandy silt and sand substrates in tide-influenced environments, which is consistent with environment properties of the type locality of the new species.

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