MEDUSAE. 33 



Not long ago Professor Hartlaub informed me in a letter that lie had obtained a 

 new Medusa from Norway with open sensory pits, but thought, after all, that it must 

 be Thaumantias pilosella of Forbes. An exchange of letters and specimens proved 

 that the specimens were Tliaumantias pilosdla, and Trof Hartlaub adopteil the name 

 Cosmetira for the genus. 



Cosnu'tiia pilosella is a very common I\Iedusa during the summer month.s on the 

 British coasts, and occa.sionally it occurs in vast shoals. 



The Medu.sa de.scribed by Prof. Maas (1893) under the name of Ihtlopsis meyalotis 

 belongs to this genus. 



I have ahso placed provisionally in this genus a new Antarctic species called 

 Cosmetira frit/iila, a description of which is given on p. 35. 



TiAROPSLS, Agassiz, 1849. 



Generic Character. — Mitrocomidae with four radial canals; with eight sensory pits ; 

 with an ocellus adjacent to each sense organ ; without marginal cirri. 



This is the oldest genus of the family, and contains about si.K specie-s. Mr. Torroy 

 (1909) has recently split the genus into two. For the species with tentacles which 

 are all alike the old name Tiaropsis is retained, but for those species with two 

 kinds of tentsicles, large and small (some of which are no doubt young stages), he 

 proposes a new genus called Tiaropsidium. The one character common to all the 

 species, and it is a conspicuous character, is the presence of a definite ocellus adjacent 

 to the sensory pit. I consider that it is best to keep all the species with this character 

 together, and that another genus is not really wanted. I must leave for another 

 occasion the critical examination of the species, which are at present as follows : 

 Tiaropsis multiciri'ata (M. Sars), 1835, T. diademata, Aga.ssiz, 1849, T. mediteiTanea, 

 Metschnikoff, 1886, T. rosea, A. Aga.ssiz and Mayer, 1889, T. punctata, Mayer, 1900, 

 T. davisi, Browne, 1902, and T. kelseyi (Torrey), 1909. 



MlTROCOMELLA, Haeckel, 1879. 



Generic Character. — Mitrocomidae with four radial canals ; with sixteen sensory 

 pits ; with marginal cirri. 



This genus probably contains only a single species, namely, M. polydiadema 

 (Romanes), 1876, which has been found on the coasts of the British Isles and Norway. 

 On a casual examination it is easy to mistake this Medusa for Cosmetira j/ilosella, 

 unless the number of .sen.se organs be counted. 1 think the species described by mc 

 in 1903 under the name of Mitrocomella fnlni had better be placed as a synonym of 

 M. polydiadema. 



MiTROcoMA, Haeckel, 18()4. 



Generic Character. — Mitrocomidae with four radial canals, with numerous open 

 sensory pits, with marginal cirri. 



