2?2 DISCOVERY REPORTS 



a species typical of warmer waters, is present in the samples nearer the surface, and C. skogsbergi, 

 typical of colder waters, is present in deeper samples. These instances are probably a result of the 

 influence of the Benguela current. 



Two previous expeditions, the 'Valdivia' (Muller, 1906) and the German South Polar (Miiller, 

 1908), obtained plankton samples from the Benguela current in latitudes similar to those of the present 

 series but further from the coast. All the species recorded here, with the exception of C. echinata and 

 C. obtusata antarctica, were recorded by these expeditions. It is of interest to compare the species 

 occurring in the Benguela current with those described from the east coast of Africa (Cannon, 1940). 

 Cannon recorded C. alata and C. atlantica as the most numerous species. The second of these is 

 totally absent from the present series of samples, though it is known to occur in similar latitudes out in 

 mid-Atlantic. 



The data given in Table 2 show the significant fact that along each line of stations, with a few minor 

 exceptions, the nearer the stations were to the coast the fewer the number of adults of each species 

 captured. At those stations where the sounding was less than 200 m., a single adult C. teretivalvata 

 was captured at Station WS 980, and at Station WS 998 (near the edge of the Continental Shelf) 

 single adults of C. elegans, C. obtusata and C. teretivalvata were taken in a 175-100 m. sample. Other- 

 wise no adults were present. The comparatively small depth of water over the Continental Shelf will 

 naturally limit the spread of those species which only occur in deeper water, though of course there 

 may be other reasons for the scarcity of Ostracods in shallow water. An absence of specimens in the 

 samples, however, does not necessarily indicate a complete absence of a species from the area in which 

 the sample was taken. A comparison of the number of adults present in the 50-0 m. and 100-50 m. 

 samples in the deeper water stations with the number in similar samples taken over the Continental 

 Shelf shows marked differences. Thus in the six 50-0 m. samples taken at stations where the sounding 

 was 932 m. or more, a total of 105 adults was found, an average of over seventeen per sample, and 

 neglecting C. elegans at Station WS 977, for reasons which will be pointed out later, there was a total 

 of twenty-two adults, an average of almost four per sample. Only two of the six samples were devoid 

 of adult specimens. On the other hand, in twelve similar samples taken at stations where the 

 sounding was 293 m. or less, there was not a single adult Ostracod. In six samples from the 100-50 m. 

 net, taken at stations where the sounding was 932 m. or more, there was a total of forty-three adult 

 Ostracods, an average of about seven per sample. Only one sample contained no specimens, while in 

 nine similar samples taken at stations with soundings of 293 m. or less, a single adult specimen was 

 found. 



It appears, therefore, that although under the conditions when the samples were taken adult 

 Ostracods occurred near the surface at stations where the water was deep, at the shallower water 

 stations over the Continental Shelf there must have been some factor or factors (connected for 

 example with temperature or water movements) which limited their spread. 



DEPTH DISTRIBUTION 

 Although Fowler's work (1909) in the Bay of Biscay is marred (as pointed out by Skogsberg, 1920) 

 by a number of taxonomic errors, it is still clear from his results that in a number of species of the 

 Halocypridae there is a vertical nocturnal migration, and that each species has an optimum depth of 

 occurrence in particular conditions. Certain species in the present series of samples occur in sufficient 

 numbers to indicate similar results. A graphical representation of the depth distribution of the adults 

 of these species is given in Figs 4 and 5. (These figures are corrected for the differing lengths of the 

 column through which the nets were hauled.) In the case of C. elegans (Fig. 4) it is clear that the 

 distribution of the species is centred around a depth of from 250 to 500 m. At Station WS 977 a large 



