ZYGODACTYLA. 103 



May not the T. gibbosa of Forbes be a young Halopsis ? They re- 

 semble the young of this .species ; also T. globosa, and perhaps T. pilo- 

 sella. We have here again one of those ever-returning questions of the 

 generic identity or difference of species, showing great structural differ- 

 ences, such as we find between these two species ; the discovery of the 

 Hydrarium will settle the point. H. cruciata, with its high bell (Figs. 

 151, 152), its four chymiferous tubes, the nature of its compound eyes, 

 and its habits, would seem to be associated with Tiaropsis, among the 

 OceanidaB ; while the tentacular cirri and the arrangement of the com- 

 pound eyes place it in the closest relationship to H. ocellata. 



Massachusetts Bay, Nahant (A. Agassiz). 



Cat. No. 374, Nahant, 1863, A. Agassiz. Medusre. 



Cat. No. 379, Nahant, June, 1862, A. Agassiz. Medusse. 



Cat. No. 445, Nahant, June, 1864, A. Agassiz. Medusae. 



ZYGODACTYLA BRANDT. 



Zygodaclyla BR. Prod. ; in Mem. Acad. St. Petersburg, p. 221. 1835. 

 Rhacostoma AGASS. ; in Proc. Bost. Soc. Nat. Hist., III. p. 342. 1850. 



Zygodactyla groenlandica AGASS. 



Zygodactyla groenlandica AGASS. Cont. Nat. Hist. U. S., IV. p. 360. 1862. 



JEquorca r/rocnlandica PER. et LES. ; in Ann. du Mas., XIV. p. 27. 1809. 



Medusa aquorea FAB. Fauna Groenlandica. No. 357. 1780. 



Rhacostoma atlanticum AGASS. ; in Proc. Bost. Soc. Nat. Hist., III. p. 342. 1862. 



^Equorca globularis MORCH. ; in Besk. af Gronland, p. 96. 1857. 



JEquorea groenlandica LESS. Zooph. Acal., p. 313. 1843. 



This species, of which a short description was given by Professor 

 Agassiz in the Proceedings of the Boston Society of Natural History for 

 1850, who supposed it to be an undescribed species, is one of the largest 

 of our naked-eyed Medusa3. It is not uncommon to find specimens meas- 

 uring as much as fifteen inches in diameter when fully extended. There 

 are in full-grown specimens from eighty to a hundred chymiferous tubes 

 (Fig. 153), with three and even four long retractile marginal tentacles 

 between every two chymiferous tubes ; the pendent membrane, which 

 forms the digestive cavity, is very contractile, having a circular opening, 

 with short lips and fimbriated edges, corresponding to the chymiferous 

 tubes, which appear to be hardly long enough, when expanded (Fig. 153), 

 to close up the edges, while at other times the lips of the actinostome 

 hang down far below the level of the circular canal, like a sheaf (Fig. 

 154), and at other times the lips hang down loosely from what seems a 

 small opening, or flare out so as to measure five or six times the diam- 

 eter of their base. The chymiferous tubes extend a short distance down 



