24 AMBKICAN HYDEOIDS. 



EucopeUa. — This genus, although very generally recognized at the present time, will have to 

 be dropped m favor of the genus Orihopyxis. EucopeUa was proposed in 1885, the type-species 

 being E. campanula da, nearly allied to the familiar Clytia caliculata of authors. Agassiz in 

 1862 based his genus Orthopyxis on his O.^oieritim, which is now considered as Clytia caliculata. 

 A further discussion of this matter will be found iii connection with the forrnal discussion of the 

 genus Orthopyxis on page 63. 



Halisiphonia. — Another of AUman's genera based on the Challenger collections.' This does 

 not come within the definition of the Campanularidse as used in this work. The original definition 

 is as follows : 



Trophosome. — Hydrocaulus a monosiphonic stolon. Hydrothecse tubiform, with entire margin, destitute of 

 operculum, vnth the cavity directly continuous with that of the peduncle or stolon, and with the hydrothecal walls 

 never adnate to the hydrocaulus. Hydranth with conical hypostome. Gonosome. — Gonangial capsules borne by the 

 hydrocaulus. 



The combination of absence of diaphragm and a conical hypostome would exclude the 

 genus from the CampamdaridtB. 



Laomedea. — Broch (1909)^ attempts to make a distinction between Campanularia and 

 Laomedea on the basis of the hydrothecal diaphragm. He shows that in Campanularia integra 

 there is a thickening of the hydrothecal walls below the hydranth, and that the apparent dia- 

 phragm is merely the basal portion of the stutzlamelle of the hydranth, while in Laomedea geni- 

 culata (Linnseus) there is a true diaphi-agm. It seems to me, however, that we have here a 

 difference in degree and not in kmd. The thickening of the walls in Campanularia integra, by 

 being accentuated and more sharply locahzed woxdd produce such a diaphragm as is shown in 

 Broch's figure of Laomedea geniculata, and is not sufficient, in my opinion, to constitute a good 

 generic character. I would therefore discard the highly unnatural geniis Laomedea (as used by 

 Broch), in which species of such very different gonosomes are included, and place the species 

 in Campanularia, Gonothyrsea, and Ohelia. 



Hehella. — This genus contains species with a well-defined diaphragm and a conical hypo- 

 stome. This latter character is sufficient to take the genus out of the family Campanularidse. 



After ehminating these genera, the following remain to be considered: Campanularia, Clytia, 

 Ohelia, Ohelaria, Thaumantias, Gonothyrsea, Silicularia, and Orthopyxis. 



It should be understood at the outset that it is clearly impossible to define the genera of 

 hj'droids on the basis of the trophosome alone, and this is particularly true of the Campanularidse. 

 Indeed, the gonosome is more useful than the trophosome in affording generic distinctions in 

 this family. A combination of the two will in most cases yield a practicable generic distinction. 



KEY TO THE GENERA OF CAMPANULARID-i;. 



Colonies always regularly branched. 



Gonangia with medusiform acrocysts which do not become free Gonothyrsea. 



Gonangia producing free medusse Obelia. 



Gonangia producing ova which develop outside Ohelaria. 



Colonies unbranched, or if branched, with an upright stem which does not spring from a creeping rootstock. 



Gonangia containing fixed sporosacs '. Campanularia. 



Colonies unbranched and pedicels springing from a creeping rootstock. 



Gonangia producing free, hemispherical medusse with eight Uthocysts at birth : Clytia. 



Gonangia producing medusie without lithocysts Thaumantias. 



Hydrothecal walls greatly thickened. 



Hydrothecse unsymmetrical, not capable of containing the retracted hydranth , .Silicularia. 



Hydrothecse usually symmetrical, capable of containing retracted hydranth Orthopyxis. 



It must be remembered that such keys as the above are merely conveniences and are not 

 intended to point out the really important generic characters. These latter will be found in the 

 definitions of the several genera. 



' Challenger Reports, Plydroida, pt. 2, 1888, p. 30. 

 ^ Die Hydroiden der arktischen Meere, p. 183. 



