562 



MEDUSA OF THE WORLD. 



Bigelow concludes that the sculpturing of the central lens of the exumbrella, the presence 

 or absence of warts, etc., constitute the best criteria for the separation of species. The furrows 

 ot the central lens are, when present, always one less in number than the tentacles. Bigelow 

 doubts the existence of A . alexanJn and finds slight furrows on the central disk of the specimens 

 which I described from Hawaii as A. alexandri. He therefore merges "A. alexandri" with 

 A. wyvillei. 



A study of the large number of specimens of Atolla in the National Museum at Washington 

 convinces me that A. bairdn and A. wyvillei are very closely related if not mere individual 

 variations of one and the same species. In some specimens the annular ridge in the outer side of 

 the ring-furrow is so narrow and indistinctly separated from the marginal zone of pedalia that 

 one hesitates to decide whether to consider it to be present or absent. Moreover the margin 

 of the central lens in all Atollas is apt to be more or less notched with faint sinuosities in its 

 outline, and thus A. bairdn and A. wyvillei may be only varieties of one and the same species. 



8 so-called species of Atolla have been described, but I believe only three can be distin- 

 guished upon non-intergrading characters. These are: 



A. bairdii Fewkes, with an annular ridge within the ring-furrow between the central lens and 



the marginal zone of pedalia. Exumbrella surface of lappets smooth. A. valdivia: and 



A. gigantea are probably identical with A. bairdn. 

 A . chum Vanhbffen has the exumbrella surface of lappets besprinkled with papillae, very wide 



central lens, and an annular ridge in ring-furrow. 

 A.u'vvillet Haeckel, with margin of central lens notched with radial furrows; no annular ridge 



in the ring-furrow. This is probably identical with A. achillis and A. verrillii. 



A synopsis of the distinctive characters ot the forms is presented in the following table: 



Synopsis of the So-called Species of Atolla. 



*These are probably all A. bairdii. 



(These are probably A. wyvillei. 



To a great extent the conditions exhibited by these so-called species of Atolla intergrade. 

 For example we can draw no sharp line ot distinction throughout the series between a smooth, 

 central lens with faint notches in its margin and as is seen in A. bairdii, and a lens deeply 

 furrowed by radiating valleys as in A. wyvillei. Similarly the wide, annular ridge, which pro- 

 jects prominently on the outer side of the ring-furrow in A. bairdii grades insensibly to the 

 condition seen in A . wyvillei wherein the ridge is so narrow that it is quite hidden by the ring- 

 furrow which over-arcbes it. With the exception of A. chuni, which stands apart, all of the 

 others may be mere local races or varieties of one variable species, A. wyvillei. 



