14 



HJALMAR BROCH. 



[REP. OF THE "MICHAEL SARS" NORTH 



If the same conditions of life always cause one and 

 the same kind of variant, we must keep this group sepa- 

 rate from the typical individuals of the species as a distinct 

 "form". Such forms may be looked at in two different 

 ways. We may either consider the forms as biological 

 or bi ogeogr a p h ic al (in the original strict sense of 

 the word), i. e. each developing stage is confined to dif- 

 ferent strata or habitats, or as zoogeographical, i 

 e. they characterize special geographical regions ("geo- 

 graphical form"). 



On these investigations the two "species" are sepa- 

 rated by referring all the specimens with smooth central 



disc, or with merely irregular traces at the margin of 

 the central disc to Atolla Bairdi, and the others to 

 A. Vet rilli. 



First let us look at the bathymetrical arrangement 

 of the developing stages (fig. 7). At first sight we might 

 think that the respective stages of A. Verrilli would, on 

 the whole, live deeper, but a careful examination of the 

 data soon disproves this; the disposition of the specimens 

 is evidently due to the uncompleteness of the material. 

 The material from the "Michael Sars" is very much richer 

 than that of any former expedition, but in spite of this 

 there are too many missing links to make the chain com- 



Station 



Bairdi 



Verrilli 



Station 



Bairdi 



Verrilli 



Station 



Bairdi 



Verrilli 



10 



9 



1 



53 



9 



5 



82 



9 



5 



23 



3 



— 



56 



2 



6 



84 



3 



1 



25 



1 



— 



62 



— 



1 



87 



7 



1 



29 



4 



4 



64 



5 



2 



88 



4 



3 



35 



— 



3 



66 



1 



— 



90 



2 



2 



42 



1 



— 



67 



2 



— 



92 



5 



1 



45 



3 



6 



70 



2 



— 



94 



10 



2 



49 



2 



3 



80 



10 



7 



98 



4 



1 



51 



1 



1 



81 



5 



5 



101 



4 



— 



Table showing the number of specimens of Atolla Bairdi and A. Verrilli at the stations of the "Michael Sars" 

 (excluding some specimens subsequently found among the plankton samples.) 



Atolla Bairdi: 



% 



Diameter of disc (millimetres) 



Total 





1-10 



11—20 



21-30 



31—40 



41—50 



51-60 



61—70 



71—80 



81—90 



91—100 



101 — 110 



61—65 



1 



2 



1 







2 



1 





_ 



_ 







7 



56-60 



5 



8 



7 



4 



5 



2 



1 



1 



1 



1 



— 



35 



51-55 



6 



13 



10 



5 



1 



1 



— 



— 



— 



— 



2 



38 



46-50 



4 



14 



2 



1 



— 



— 



— 



— 



— 



— 



— 



2! 



41—45 



— 



6 



— 



— 



— 



— 



— 



— 



— 



— 



— 



6 



Total 



16 



43 



20 



10 



6 



5 



2 



1 



1 



1 



2 



107 



Atolla Verrilli: 





Diameter of disc (millimetres) 





% 





Total 



1—10 11—20 



21-30 



31-40 



41—50 



51-60 



61-70 1 71— 80 



81-90 



91—100 





£6-60 





5 



3 



1 



3 



3 



1 



1 



1 



1 



19 



51-55 



1 



12 



6 



3 



— 



— 



2 



1 



1 



— 



26 



46—50 



— 



8 3 



1 















12 



41-45 



— 



2 1 



— 



— 



— 



— 



— 



— 



— 



3 



Total 



1 



27 



13 



5 



3 



3 



3 



2 



2 



1 



60 



Fig. 8. Tables showing the diameter of the central disc in the "Michael Sars" specimens of Atolla Bairdi and 



A. Verrilli and the relative size of (he central disc referred to the total diameter of the medusa (without lappets) 



expressed in percentages (°/o). The figures denote number of individuals. 



