238 DISCOVERY REPORTS 



where they predominated. Even at the intervening stations they ranked second only to the chaeto- 

 cerids. The unavoidably heterogeneous nature of the group, however, was very evident. Whereas 

 inshore the strongly neritic species Fragilaria karsteni predominated, along with Asterionella japonica, 

 it was the oceanic Thalassiothrix longissima and panthalassic Nitzschia delicatissima that accounted for 

 nearly all of the group totals farther out. 



The mid-intermediate line proved too short to extend beyond the rich coastal diatom zone. The 

 estimated diatom totals were high throughout and very closely paralleled by the group-totals for the 



WS986 



-D 



d 



D 7 



d 



d 



D 4 

 d 



"W5990 WS|Wl WS992 WSW WS994 WS995 



• • TOTAL DIATOMS 



-♦ CHAETOCERACEAE 

 ■ -' "PENNATAE' 

 ». BIDDULPHIINEAE 

 o DISCINEAE 

 — SOLENIINEAE 

 \ '" 

 \ 

 \ 



\V 



B 



X) 



IOO 



SEA MILES FROM LAND 



F'g- 75- Estimated total diatoms, and diatom group totals, survey I. A. Sylvia Hill line, io-ii March 1950. 



B. Southern Intermediate line, 11-12 March 1950. 



predominating chaetocerids. The other groups clearly fulfilled a minor role, much on the same level, 

 with 'Pennatae' of some consequence inshore. Apart from the profusion of neritic Chaetoceros 

 species, at least ten in number, the species of most importance were : 



Among the Discineae, inshore, Stephanopyxis turris diminishing rapidly as one proceeded seawards; 

 and Thalassiosira sabtilis, which reached its highest numbers and relative importance at station 

 WS 983. 



Among the Biddulphiineae, Eucampia zoodiacus at the two inshore stations only. 



Among the Soleniineae, Dactyliosolen mediterraneus and much lesser numbers of the more cosmo- 

 politan Rhizosolenia spp. farther offshore. 



Among the ' Pennatae ', Fragilaria Karsteni and Asterionella japonica inshore with Thalassiothrix 

 longissima at station WS 984. 



On the Sylvia Hill and southern intermediate lines, diatom distribution followed an essentially 

 similar pattern. Very high estimated totals were recorded inshore, with an abrupt decrease near the 

 shelf-edge some 60 sea-miles from the land, and a levelling out to very low values towards the sea- 



