BROWNE— MEDUSA FROM THE INDIAN OCEAN 203 



Genus Atolla, Haeckel, 1880. 



After reading the work of Bigelow (1909 and 1913), Mayer (1910) and Broch (1913) 

 on Atolla, it seems to me that the genus has only two species, namely, Atolla wyvillei 

 and Atolla chuni. The latter is easily distinguished by the presence of warts on the 

 marginal lappets. All the other species which have been described by various authors 

 apparently fall under the oldest name of Atolla wyvillei. The presence and shape, or the 

 absence, of radial furrows on the central disc, formerly relied upon for distinguishing 

 species, are apparently worthless as specific characters. 



Broch> after examining over 200 specimens oi. Atolla collected by the "Michael Sars" 

 in the North Atlantic, was able to separate the material into three groups. 



" First, those furnished with distinct radial furrows all over the central disc ; second, 

 those with incomplete radial furrows, in many cases visible only at the margin of the 

 central disc ; and third, those with a perfectly smooth central disc, showing no trace 

 whatever of radial furrows."..." The intermediate group contains every transition stage 

 from Atolla hairdii (with a smooth central disc) to Atolla verrillii (with narrow radial 

 furrows on the central disc), and we are therefore compelled to consider Atolla verrillii as 

 a synonym of Atolla hairdii." 



Out of this large number of Atolla Broch was able to isolate a single specimen, 

 which he has placed under the name oi Atolla ivyvillei. "The broad and conspicuous 

 radial furrows of the central disc and the strongly-marked longitudinal furrows of the 

 pedalion distinguish this species from the other Atlantic species." Broch apparently 

 found no connecting link between Atolla ivyvillei and Atolla verrillii. 



Bigelow, according to his reports on collections from the Eastern and North- Western 

 Pacific regions, has experienced the difficulty of separating Atolla ivyvillei (with broad 

 radial furrows) from Atolla verrillii (with narrow radial furrows). He says: "In the 

 Eastern Pacific specimens there was considerable variation in the breadth of the furrows, 

 which were usually broad in large, narrow in small specimens." In the North- Western 

 Pacific specimens " the furrows vary so much that no sharp line can be drawn between 

 specimens in which they are broad and those in which they are narrow." 



Bigelow records no specimens in these collections with a perfectly smooth disc 

 {Atolla hairdii). It is clear from Broch's researches that the absence of or presence of 

 radial furrows are not good characters for the determination of species, and from Bigelow's 

 researches that the width of the radial furrows is also useless. 



Broch also gives tables which show that there is no good evidence for assuming that 

 the smooth-disc form o? Atolla and the furrow-disc form are separate geographical races, 

 or that they live at different depths, or that differences are due to growth. The two 

 kinds are found together at the same stations, depth, and of the same size. 



44. Atolla wyvillei, Haeckel, 1880. 



Localities. N.E. of Chagos, Lat. 4° 30' S., Long. 71° 15' E. (Depth about 2000 fms.) 

 2 specimens caught in Fowler's self-closing net. 1000 — 500 fms. 18 May, 1905, J. 

 Between Providence and Alphonse, Lat. 8" 16' S., Long. 51° 26' E. 1 specimen caught 

 in a large square net. 900 — fms. 6 Oct. 1905, aa. 



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