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 tor the separation ot species. The turrows 

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

 doubts the existence of A. alexandn and finds slight turrows on the central disk of the specimens 

 which I described trom Hawaii as A. alexandn. He therefore merges "A. alfxariJii" with 

 A. wyvillei. 



A stud)' of the large number of specimens ot Atolla in the National Museum at ^^ ashington 

 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 indistinctlv separated trom 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. hatrdii and A. ivyvtllet may be only varieties of one and the same species. 



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

 guished upon non-intergrading characters. These are: 



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



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



A. gigantca are probably identical with A. bairdii. 

 A . chum VanhofFen has the exumbrella surface of lappets besprinkled with papilhr, very wide 



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

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



in the ring-furrow. This is probably identical with A. achilhs and A. verrillti. 



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



Synopsis of the So-called Species of Atolla. 



•These are probably all A. bairdii. 



■fThese 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 of distinction ihroughout the series between a smooth, 

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

 furrowed b\' radiating valle\s as in A. wyvilli-i. Similarly the wide, annular ridge, which pro- 

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

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

 furrow which over-arches it. With the exception of//, chuni, which stands apart, all of the 

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



