PTYCIIOGENA. 137 



younger stages. (Figs. 215 a , 216 a , 217.) Fi =- 219 - 



This Medusa grows to a large size, meas- 

 uring often six to eight inches in diameter. 

 It is one of the earliest Medusae to make 

 its appearance, attains its full size rapidly 

 from May to June, and by the end of 

 June the dead Medusas are found in large 

 numbers, floating about after storms ; by 

 the middle of July they have all disap- 

 peared. Found at Nahant. 



Massachusetts Bay (Agassiz) ; Maine (Stimpson). 



Cat. No. 275, Nahant, A. Agassiz. Young and old Medusae. 



Cat. No. 359, Boston Harbor, L. Agassiz. Medusa. 



Staurophora Mertensii BR. 



Staurophora Mertensii BR. ; in Mem. Acad. St. Petersb., IV. p. 400, Pis. 24, 25. 1838. 

 Staurophora Mertensii LESS. Zooph. Acal., p. 297. 1843. 



Norfolk Sound (Mertens). 



PTYCHOGENA A. AGASS. 



The Medusa for which this genus has been established shows the 

 intimate structural connection between Staurophora, Melicertum, and 

 Polyorchis. The structure of the genital organs is an intermediate 

 state of development between organs where the folds of the actinos- 

 tome are lost in the genital folds, as in Staurophora, and the other 

 extreme, where we have pendent genital organs attached to one ex- 

 tremity of diverticulate chymiferous tubes, as in Polyorchis. 



Ptychogena lactea A. AGASS. 



The bell of this Medusa is rather high (Fig. 220), and the sphero- 

 some of considerable thickness, giving this species an appearance of 

 consistency, which is heightened by the striking contrast with the 

 water of the milky genital organs and numerous marginal tentacles. 

 The chymiferous tubes are broad ; at an equal distance on the abac- 

 tinal and actinal ends, the edges of the tube become hacked ; the 

 notches increase in size, and soon become long, sharp folds of the 

 walls of the chymiferous tubes, projecting at right angles from the 

 tubes (Fig. 221) ; the larger of these folds branch again. To these 

 folds the genital organs are attached, forming as many connecting 



Fig. 219. Young Staurophora, having the general aspect of the adult. 



NO. II. 18 



