Observations on British and Norwegian Hydroids and Their Medusae

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Abstract
The newly liberated medusa of Perigonimus muscoides M. Sars has been found to be a Bougainvillia. A specimen from the plankton points to the extreme probability that it can be regarded as synonymous with a previously described medusa, B. nordgaardi (Browne). The name of both hydroid and medusa now becomes B. muscoides (M. Sars 1846).The systematic importance of the above discovery is discussed, and the genus Rhizorhagium M. Sars 1877 is redefined to include all those species of “Perigonimus” with fixed gonophores, viz. Rhizorhagium roseum, R. sarsi, R. antarcticum, R. formosum, and a new species, R. album, from the Cornish coast.Variations in the form of the hydroid of Leuckartiara octona are described. The newly liberated medusa of Perigonimus abyssi G. O. Sars is figured for the first time and the species referred to the genus Leuckartiara.A new Bimeria-like hydroid, Thamnostoma russelli, is described. The characters of the medusa indicate that it belongs to the genus Thamnostoma Haeckel, and the characters of the hydroid show that the genera Thamnitis and Thamnostoma must be united.Corymorpha annulicornis M. Sars is redescribed from living specimens and the medusa is shown to be Euphysa aurata Forbes. It must henceforth be known as Corymorpha aurata (Forbes).A new species of Campanulina, C. paracuminata, with its young medusa is described from Plymouth.Notes are included on Boreohydra simplex, Stauridium productum and Staurocoryne filiformis.Certain points in the synonymy of all these species are discussed.

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