GEOGRAPHICAL DISTRIBUTION. 359 



Except for the difference between the cold waters close to the coastal slope, and 

 the warmer regions to the west, the entire area explored was remarkably uniform 

 so far as its qualitative Siphonophore fauna was concerned. The holoplanktonic 

 surface Medusae showed no evidence of any faunal division (Bigelow, :09). 

 On the contrary all the surface Medusae which occurred regularly enough for 

 me to plot their distribution, were met with throughout the entire range of 

 surface temperatures which we encountered. 



Quantitative distribution. The interesting feature from this point of view is, 

 of course, the conditions in the Humboldt Current and the equatorial flow on 

 the one hand, and in the desert area on the other. The Siphonophores, like 

 the Medusae and other pelagic organisms, illustrate the difference between the 

 two. To show the comparative richness of the one and the poverty of the 

 other, I have reckoned up the average number of specimens taken per haul, to 

 the northeast and to the southwest of the dividing line between rich and poor 

 surface fauna as plotted by A. Agassiz ( : 06, pi. 3c) for several of the commoner 

 species, with the following results. The best illustration is afforded by the polj-- 

 gastric state of Abylopsis tetragona, because this species is so large and conspic- 

 uous that I was able to pick out all, or nearly all, the specimens in each haul, even 

 when the catch was a solid mass of Salpae, pteropods, etc. In the case of the 

 smaller Diphyids, on the contrary, many specimens were apt to be overlooked 

 in rich hauls, while in the poor ones every example of every species was easily 

 separated.^ In the case of Abylopsis tetragona the average number per haul 

 in the rich region of the Current was 19, per haul in the barren region, 2. In 

 other words the proportion are roughly as 9.5 to 1. But in reality the discrep- 

 anc}^ was much greater, because a swarm was encountered at one Station 

 (4052) in the Current, and no swarms were met with elsewhere. 



Agalma okeni affords a second example. Leaving out of consideration the 

 cold axis of the Current where it did not occur at all, we find that the proportion 

 between specimens per haul, "Current" and "extra Current," is about 13 to 1. 

 And in the case of this species, the discrepancy between the desert and the 

 total remaining area traversed is much greater than it is for Abylopsis tetragona, 

 for out of a total of upwards of eighty colonies, only one was taken in the barren 

 region. 



Other common species, e. g., Diphyes appendiculata, D. bojani, Diphyopsis 



' In this estimate I consider only our lines Galapagos — Callao — Easter Island — Manga Reva. 

 The results are obtained by dividing the total number of specimens by the total number of hauls, made 

 in the corresponding region. 



