This updated re-description of the prayine siphonophore Rosacea cymbiformis includes figures of all zooids (except larval nectophores) and is based on material held in the collections of the Natural History Museum (NHM), London. Rosacea cymbiformis was originally described in 1830 under the name Physalia cymbiformis, and subsequently reported many times during the 19th Century. However, during the 20th Century it was confused with the closely related species R. plicata, and the two species are still not clearly differentiated. Previous descriptions are reviewed herein, including conflicting interpretations of nectophore designation in R. plicata, and bract orientation in R. cymbiformis and R. plicata. To identify these siphonophores to species level and separate them from other closely related prayines, it is essential to distinguish the first definitive nectophore from the second, and the right paired bracteal canals from the left canals. This becomes critical when only detached siphonophore zooids are available, as for example, in plankton samples collected with nets. A summary of the differences between R. cymbiformi and the five other currently recognized Rosacea species, R. plicata, R. repanda, R. limbata, R. flaccida and R. arabiana, is presented. The full synonymy of R. cymbiformis is too long for inclusion here and is deferred to a later paper.
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