MEDUSA. 83 
Not long ago Professor Hartlaub informed me in a letter that he 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 T’haumantias pilosella, and Prof. Hartlaub adopted the name 
Cosmetira for the genus. 
Cosmetira pilosella is a very common Medusa during the summer months on the 
British coasts, and occasionally it occurs in vast shoals. 
The Medusa described by Prof. Maas (1893) under the name of Halopsis megalotis 
belongs to this genus. 
I have also placed provisionally in this genus a new Antarctic species called 
Cosmetira frigida, a description of which is given on p. 35. 
Traropsis, Agassiz, 1849. 
Generic Character.—Mitrocomide 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 six species. Mr. Torrey 
(1909) bas recently split the genus into two. For the species with tentacles which 
are all alike the old name Yvaropsis is retained, but for those species with two 
kinds of tentacles, 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 multicirrata (M. Sars), 1835, T. diademata, Agassiz, 1849, T. mediterranea, 
Metschnikoff, 1886, 7. rosea, A. Agassiz and Mayer, 1889, 7. punctata, Mayer, 1900, 
T. davisi, Browne, 1902, and T. kelseyi (Torrey), 1909. 
Mirrocometia, Haeckel, 1879. 
Generic Character.—Mitrocomide with four radial canals; with sixteen sensory 
pits; with marginal cirri. 
This genus probably contains only a single species, namely, MM. 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 pilosella, 
unless the number of sense organs be counted. I think the species described by me 
in 1903 under the name of Mitrocomella fulva had better be placed as a synonym of 
M. polydiadema. 
Mirrocoma, Haeckel, 1864. 
Generic Character.—Mitrocomide with four radial canals, with numerous open 
sensory pits, with marginal cirri. , 
