BATHYMETRIC RANGE. 229 



Although it is now generally admitted that, in spite of occasional records 

 of the sporadic appearance of such genera as Atolla and Periphylla on the 

 surface, they are not normally surface forms, the question at what par- 

 ticular zone they are most abundant, or whether they are indifferently dis- 

 tributed from top to bottom, or even live just above the bottom, as recently 

 suggested by Maas (: 04"), has not as yet been answered conclusively. 

 In general two views have been held, though not, it is true, always 

 clearly defined : first, that they belong for the most part to abyssal depths, 

 as both Maas (: 04", : 05) and Vanhoffen (: 02") have in general main- 

 tained ; and secondly, that they reach their greatest abundance in the upper 

 layers, between the surface and a depth of about 300 fathoms. Discussions 

 of this question have, for the most part, been rather inadequate, due 

 largely to the use of the much-abused term " deep sea," largely to differ- 

 ences of opinion as to what constitutes " great depths." And inasmuch as 

 the location of the normal range of this group of organisms is of much 

 interest in the general ecology of the intermediate fauna, a re-examination 

 of the evidence, most of which unfortunately is indirect, is desirable. 



In so far as concerns the occurrence of intermediate Medusae on the sur- 

 face, it is to be noted that although both Atolla and Periphylla have been 

 taken there, none of the recent exploring expeditions have captured any of 

 this group on the surface, with the exception of one fragmentary specimen 

 of Halicreas papillositm in the present collection, in spite of the great number 

 of surface hauls which have been made both by day and by night ; on the 

 other hand, all recent expeditions have taken intermediate Medusae in con- 

 siderable numbers in vertical and intermediate hauls. Probably, then, the 

 occurrence of these genera at the surface is to be interpreted, not as an 

 indication of their normal habitat, but as the manifestation of some such 

 physical phenomenon as unusually violent vertical circulation of the water, 

 or a peculiar condition of oceanic temperatures. 



The most important direct evidence to show the exact depth from which 

 individual specimens were taken is of course that afforded by closing-net 

 hauls. Unfortunately, however, the captures of Medusae in such hauls have 

 been comparatively few, the following being the only records which I have 

 been able to find: — on the "Valdivia" expedition, so far as the results of 

 Vanhoffen's researches are yet published. Medusae were taken but three 

 times in closing nets; namely, one specimen of Periphylla at 810-540 fath- 

 oms (1,500-1,000 meters), one specimen of Colobonema at 810-485 fathoms 



