4 86 DISCOVERY REPORTS 



As an indication of the actual hydrological conditions I have noted the bottom 

 temperatures where known. 1 Much information on the general circulation is to be found 

 in Deacon's report (1937). In particular his section 9 (pis. xvi-xviii), which runs from 

 the Antarctic to Australia across the South Indian Ocean, is instructive in this connexion. 

 His sections afford ample evidence that in the relatively shallow parts of the oceans the 

 bottom fauna may be immersed in a water-layer which in deeper parts is an intermediate 

 layer. 



It thus seems possible, though the evidence is insufficient and not conclusive, that 

 the apparent discontinuities in the distribution of these abyssal forms are significant in 

 that they may represent a real absence of the species under certain hydrological con- 

 ditions ; but are fortuitous in that the depths of the stations examined have happened 

 to be such that all the specimens from the same geographical region have come from 

 the same hydrological conditions. It seems possible for example that stations in deeper 

 parts of the sub-Antarctic (say in the South Indian Ocean) might yield samples of the 

 fauna of the Antarctic bottom water, thus filling the geographical gap between the 

 Antarctic and Atlantic records of these species. This would also show that, as might 

 be expected, the change in the abyssal fauna at the Antarctic convergence is less simple 

 than it appears to be in Table 3 A. 



Thus, although no conclusions can be drawn from the very scanty faunal data at 

 present available, there appear to be distinct indications that interesting results may be 

 obtained when further collecting can be done in the abyssal regions. 



Table 4 shows the distribution, in relation to depth, of the species from the Pata- 

 gonian and Magellanic regions, the stations being arranged in order of mean depth. 

 The species form three groups: (I) shallow-water species, not found below 118 m. ; 

 (II) species found at all depths; (III) species only found below 200 m. As the deepest 

 station is at only 415 m., group III does not represent an abyssal fauna, or anything 

 approaching it, but it does comprise species that have not been found on the continental 

 shelf, which is at about 180 m. Fig. 60 shows the distribution of these stations in rela- 

 tion to the 200 m. line. 



It is striking that, with two exceptions, the species restricted to the deeper water are 

 new species and varieties. Of the two known forms, Amastigia crassimarginata has not 

 been recorded elsewhere, and has no obviously near relative; A. gaussi is otherwise 

 restricted to Antarctic waters; and three of the other four species in group III and the 

 last two species in group II appear to have Antarctic affinities as the following analysis 



1 I have been unable to ascertain the bottom temperatures at the Siboga stations, but the Snellius Expedi- 

 tion took temperatures in the same region (Riel, 1934). These were published in the form of potential 

 temperatures in which allowance was made for compression (Riel, 1934, p. n). The following Snellius 

 sections give some idea of probable conditions at two of the Siboga stations : 



Section 21, Riel, p. 43 (for Siboga St. 211, south of Celebes), potential temperature at 1100 m. 



a little above 4-0° C. 

 Section 25 (Indian Ocean end), Riel, p. 47 (for Siboga St. 295, south of West Timor), potential 

 temperature at 2000 m. about 2-35° C. 

 Section 28, Riel, p. 51, passes through the region of Siboga St. 170, between New Guinea and Ceram, 

 but the temperatures are only given from 1200 m. downwards. 



