ANTARCTIC AND SUBANTARCTIC MOLLUSCA: 

 PELECYPODA AND GASTROPODA 



COLLECTED BY THE SHIPS OF THE DISCOVERY COMMITTEE 



DURING THE YEARS 1926-1937 



By A. W. B. Powell, f.r.s.n.z. 



Auckland Museum 

 (Plates V-X; Text-figs. A-N) 



INTRODUCTION 



The material covered by this report is largely from the American Quadrant of the Antarctic and 

 Subantarctic Zones, but with the addition of odd dredgings from Bouvet Island, Marion Island 

 and the Ross Sea. My report covers the Gastropoda (with the exception of the Pteropoda and the 

 Nudibranchiata) and some Pelecypoda. The Pteropoda were reported upon by Anne L. Massy (1932) 

 and the Cephalopoda (in part) by G. C. Robson (1930). 



The areas best represented by the Discovery collections are the Falkland Islands and the Patagonian 

 Shelf, South Georgia and the higher latitudes of the Scotia Arc from the South Shetland Islands to the 

 Palmer Archipelago. 



The varied nature of these respective areas is ably described by E. Heron-Allen and A. Earland 

 (1932) and A. Earland (1934) in their outstanding work on the Foraminifera. A description of the 

 physical characteristics of each of the biogeographic areas concerned with the present collections follows. 



FALKLAND ISLANDS 

 The Falklands are a group of two large and many small islands lying upon the very extensive East 

 Patagonian Continental Shelf, but almost severed from it by the transverse Falkland Trough which lies 

 at a depth of from 150 to 200 m. between two tongues of deep water which impinge both from the 

 north and the south. The Falklands lie within the Subantarctic Zone of surface waters (isotherms 

 between 6 and 12 C), and are outside the northern limit of pack-ice. They are strongly influenced 

 by the Cape Horn Current, composed largely of water of Pacific origin, which is swept through Drake 

 Strait by the West Wind Drift and then turns northwards to the Falklands and resolves into the 

 Falkland Current, which continues northward between the Falklands and Patagonia. The West Wind 

 Drift proper passes well to the south of the Falklands. Owing to its position upon the Patagonian 

 Shelf the Falkland marine molluscan fauna is predominantly Magellanic, and the terrestrial fauna, 

 notably the presence of the fresh-water genus Chilina, points to a former land link with the Patagonian 

 mainland. 



North of the Falklands at the limit of the Continental Shelf, the bottom descends steeply to the 

 Argentine Basin, which comes within the influence of the warm Brazilian Current, which there has 

 a seasonal temperature range of from 11-5 to 14-5° C. (Hart, 1946, p. 243). 



Another important factor influencing the Falkland fauna is the presence of the Atlantic-Indian 

 cross-ridge which runs from the Argentine Basin almost to the Kerguelen-Gaussberg (radial) ridge and 

 forms the northern boundary of the Atlantic-Antarctic Basin. 



Deep water to the south effectively separates the Falklands from the Burdwood Bank and the rest of 

 the extensive Scotia Arc. 



