g MEDUSA. I. 



irregular manner, all branches communicating with the circular vessel. The gonads are developed upon a number of the radial 

 canals adjacent to the stomach. More than loo marginal tentacles, each carrying an ocellus on the inner side of the bulbous 

 base. Velum is well developed. 



North-Atlantic coasts of Europe and off Newfoundland. 



Family Mitrocomidae Torrey. 



Leptomedusse with open marginal vesicles. 



The first author who has paid attention to the systematical importance of the open marginal 

 vesicles is E. Metschnikoff (1886 a, p. 5; 1886 b, pp. 81 ff.). He separated Halopsis occUata Agassiz 

 from ihe ^-Equorida- (among which it had been placed by A. Agassiz and Haeckel) and the genera 

 Tiaropsis and Ah'trocoiiia from the E/icopzdcc, and united the said forms into a family La/ocidcr. 

 Metschnikoff had demonstrated that the planulse of "-Laodicc cruciatd' as well as of Mitrocoma 

 anna developed into hydroids, exactly resembling Cuspidclla Hincks. At the first sight this seems 

 rather peculiar; but Metschnikoff calls attention to the fact that Laodicc is an Ocellate, Mitrocoma 

 a. Vesiculate, while Tiaropsis forms the connecting link between the two. If we regard Tiaropsis as a 

 more primitive form of the Lafoeidcr^ the ThaiiviantidcT (to which belongs Laodice) and the medusae 

 with open marginal vesicles have to be regarded as two diverging branches of the same group. 



Ma as (1893, p. 60) amends the lm\\\\y Lafocida: sensu Metschnikoff, including Tiaropsis, Mitro- 

 coma, Phialis (i. e. Ho/opsis critciata Agassiz), Halopsis occUata, and perhaps Eitchilofa and Mitro- 

 comella. 



Torrev (1909, p. 16) proposes the name of MitrocoiiiidcE for this family, because the medusae 

 in question bear no relation to the h}droid-family I^afocidce. The name Mitrocoriiida- is also used by 

 Browne (1910, p. 32), who gives a revision of the genera of the group and announces a critical revi- 

 sion of the species. Browne hesitates to refer Halopsis to this family, until its marginal vesicles 

 have been thoroughly examined. Later Bigelow (1914a, p. 102) has demonstrated that Halopsis 

 ocellata has open sensory pits of the type of the Mitrocomidcr. Bigelow (1913) also unites the 

 leptomedusae with open marginal vesicles into a family Mitrocomidcr, whereas Mayer (1910) does 

 not apply more systematical importance to the open marginal vesicles than that of a generic 

 character. In the quoted paper (1913) Bigelow demonstrates that ''Laodicc ccllnlaria" A. Agassiz has 

 open marginal vesicles and accordingly belongs to the Mitrocoiiiida-. As this species has many margin- 

 al vesicles but is destitute of cirri, it makes a proper genus, Halistaiira nov. — As generic charac- 

 ters Browne uses the presence or absence of cirri or ocelli together with the number of marginal 

 vesicles. Thus the genus Mitrocomclla is only separated from Mitrocoma by the number of vesicles 

 being constantly 16, whereas in the full-grown Mitrocoma the number exceeds 16. This does not seem 

 to me to be sufficient reason for a distinction of genera. The species polydiademata (the onl>- species 

 of Mitroconiella hitherto known) does not differ from the species of the genus Mitrocoma in any im- 

 portant characters, and I prefer, therefore, to refer it to that genus, following Mayer (1910, p. 290). 



A synopsis of the genera of the famih- Mitrocomidic will now look as represented in Table VI: 



