HYDROMEDUSAE 281 



whereas P. tenuis attains a height of 2 mm. ; provisionally, therefore, it will be advisable 

 to regard them as separate species. 



Genus Rgithkea Brandt (1837) 



Generic characters. Margelidae with marginal tentacles arranged in eight groups 

 (four perradial and four interradial) ; the four corners of the mouth drawn out so as to 

 form four oral arms with clusters of nematocysts. 



Rathkea formosissima (Browne) (Plate XIV, fig. 5; Plate XIX, fig. i) 



Lizzia formosissima Browne, 1902, p. 278. 

 Rathkea formosissima Mayer, 1910, p. 177. 

 Lizzia formosissima Hartlaub, 191 1, p. 144. 



Specific characters. Adult: Umbrella bell-shaped, a httle higher than broad, with a 

 slight transverse constriction level with the top of the subumbrella cavity and a solid 

 mass of jelly above it. Stomach small, somewhat cubical, about as long as broad, and 

 situated on a broad peduncle about as long as the stomach. Mouth with a plain simple 

 margin, quadrangular in shape; oral arms four, each with seven to eleven clusters of 

 nematocysts arranged in a double row with always a single terminal cluster. Gonads on 

 the stomach, four interradial roundish swellings or masses. Medusa buds may be 

 present on the stomach, interradially situated. Tentacles, five in each perradial group 

 and three in each interradial group. Colour: Stomach brownish ; compound basal bulbs 

 dark brown or black (in formalin). Size: Umbrella 3 mm. in height and 2\ mm. in 

 width (largest specimens). 



Early stage: Umbrella bell-shaped, about as high as broad, without a solid mass of 

 jelly above the umbrella-cavity. Stomach small, on a broad but very short peduncle. 

 Oral arms each with three to five clusters of nematocysts, arranged in a double row and 

 always with a single terminal cluster. Medusa buds on the stomach, interradial, usually 

 four. Gonads not developed. Tentacles, three or more in each perradial group and 

 three in each interradial group. Size: Umbrella 1-2 mm. in height and width. 



The collection contains twelve specimens taken in Stanley Harbour on 10 November 

 1898 and numerous specimens from the same locality, 6-1 1 November 1901 ; in addition, 

 one specimen taken at Port Egremont 28 October 1909 during Vallentin's third voyage. 



In the early stages the umbrella (about i mm. in diameter) is rather thin, and the 

 large solid mass of jelly above the top of the subumbrella cavity is absent. This mass of 

 jelly begins to develop during the intermediate stage and increases in size as the medusa 

 grows. In the adult it is about one-third of the umbrella. The velum is much broader 

 in the earlier stages than in the adult. 



The stomach is somewhat cubical in shape, and its peduncle increases in length with 

 the growth of the umbrella. In the early stages it is only just visible, but in the adult it is 

 about the same length as the stomach. 



The mouth tube below the stomach is fairly short, somewhat cross-shaped in a 

 transverse section ; its four perradial edges are somewhat thickened, containing a row of 

 large, vacuolated cells, and these thickened strings are prolonged beyond the four 



