occurring outside this area. These species are all Arctic-Boreal 

 and probably limited to the perimeter by their temperature 

 tolerance, but their preference for gravelly sediments probably 

 accounts for their further restriction to the Northeast Peak. A 

 group of four species represented by single occurrences on the 

 Northeast Peak are also listed in Table 1 as other species which 

 might show this distribution pattern if more data were available. 



Perimeter 



A group of 17 species (Table 1) was distributed around the 

 perimeter of the bank. The bottom temperatures around the 

 perimeter generally have an annual range between 4° and 13°C, 

 being warmer in the Great South Channel and cooler along the 

 Northern Edge. The sediments of the perimeter are fine sands 

 except for gravel on the Northeast Peak and medium to coarse 

 sand in the Great South Channel. It is noteworthy that all 17, 

 although having boreal distributions, are reported south of 

 Georges Bank (Dickinson et al. 1980) in contrast to the species 

 restricted to the Northern Edge and Western Basin. It seems 

 likely that this group of species is excluded from the central 

 parts of Georges Bank by the warmer summer temperatures 

 (14°-18°C) and greater annual temperature range (3°-I8°C) 

 found there. It is clear that sediment type may also play a role in 

 restricting the distribution of some of these species since they are 

 absent from the Northeast Peak or Great South Channel por- 

 tions of the perimeter. 



Western Basin 



A group of 14 species (Table 1) was restricted to the Western 

 Basin and Northern Edge. This area is characterized by the cold- 

 est year-round temperatures (4°-6°C) and the finest sediments 

 on the bank. Three species in the group (Haploops lubicola, 

 Stegocephalus inflatus, and Tmelonyx cicada) had single 

 records on the Southern Edge of the bank, but their distribu- 

 tions seemed to be better described by a "Western Basin" pat- 

 tern than a "Perimeter" pattern. Only 2 of the 14 species 

 occurred south of Georges Bank, and they (Ampelisca declivita- 

 tis and Tmelonyx cicada) were reported only from bathyal 

 depths (Dickinson et al. 1980). Since these records constitute the 

 southern limit for most of these species, we suspect that temper- 

 ature limits their distribution on the bank to this area of cold 

 year-round bottom temperatures. An additional group of four 

 species are also listed in Table 1 as other species which might 

 show this pattern if more data were available. 



Shoals 



Only a small group of species (Table 1) occurs in the gravelly 

 sands of the northern shoals. This area of the bank is character- 

 ized by strong currents that continually rework the sediments. 

 The shoals also experience the greatest annual temperature 

 range (4°-16°C). Three of the four species found in the shoals 

 are known to be associated with coarse sands, and they are 

 usually found in shallow turbulent sand communities of the 

 open coast. Pontogeneia inermis is a pelagic-epibenthic species 

 which is ubiquitous over the central parts of Georges Bank. It is 

 probable that other gammarideans may be associated with the 

 shoals, but we may have missed them in our limited collections 

 from this part of the bank. 



Southern Edge 



A group of eight species (Table 1) was restricted in their 

 distribution to the Southern Edge of the bank. This area is char- 

 acterized by fine sands and a narrow annual temperature range 

 fluctuating between 8° and 11°C. Since these species are all 

 cold-water forms, it seems Hkely that distribution around the 

 perimeter of Georges Bank is Umited by their preference for fine 

 sands. Three additional species are listed in Table 1 since we sus- 

 pect they might show this pattern if more data were available. 



Central 



A group of 19 species (Table 1) is associated with the medium 

 sand sediments and warmer summer temperatures (12°-14°C) 

 of the central portions of the bank. However, this group of 

 species seems to avoid the northern shoals area of the bank. 

 These species all have Virginian distributions and are endemic to 

 the western Atlantic. Some species in this group such as mem- 

 bers of the family Haustoriidae have limited distributions on the 

 bank, probably reflecting their need for special sediment charac- 

 teristics. Other species in this group such as Unciola irrorata and 

 Monoculodes edwardsi are broadly distributed in the central 

 region of the bank. In general, we suspect that this species group 

 is restricted from the perimeter of the bank by the cold year- 

 round temperatures and from the shoals by the turbulence. 



ACKNOWLEDGMENTS 



We thank our associates at the Northeast Fisheries Center in 

 Woods Hole, particularly Bruce Burns, Gilbert Chase, Evan 

 Haynes, Tom Morris, Betty Murray, Ruth Stoddard, and Henry 

 Jensen for assistance in collecting and processing samples; 

 Roger Theroux and Jacqueline Murray for their help in data 

 processing; John Lamont for his assistance in drafting the 

 figures; and Richard Langton for his critical reading of the man- 

 uscript. We would also like to thank E. L. Bousfield and Diane 

 Laubitz of the National Museum of Canada, Ottawa, for their 

 taxonomic assistance. 



LITERATURE CITED 



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