Ockelmann (1958) mentioned in his report on the Arctic 

 bivalves "In all probability, T. flexuosa has a boreal- 

 lusitanian main distribution and is absent from arctic waters." 



The reported depth range for this species is from 4 to 2,006 

 m (Clarke 1962). 



Our samples are from depths which range between 16 and 

 1,550 m with a mean of 170 m. In terms of depth range 

 groupings the majority of both samples and specimens are 

 more plentiful in the mid-continental shelf to upper slope 

 water depth ranges, with 31% of the samples and 59% of the 

 specimens in the 50-99 m grouping, and 34% of the samples 

 and 17% of the specimens in the 100-199 m grouping; the 

 200-499 m depth range grouping contains 22% of the samples 

 and 13% of the specimens. Significantly smaller amounts 

 occur on either side of these depth ranges, with 1% of the 

 samples and 0.2% of the specimens in the 0-24 m grouping, 9% 

 of the samples and 7% of the specimens in the 25-49 m group- 

 ing, 3% of the samples and 4% of the specimens in the 500-999 

 m grouping, and 1% of the samples and 0.1% of the specimens 

 in the 1,000-1,999 m grouping (Table 150). 



Thyasira flexuosa specimens were found in all sediment 

 types considered in this report; however, they were most 

 abundant in the medium-coarse to fine grained sediments with 

 smallest amounts occurring in the coarser grained fractions. 

 Silty sand substrates contained 20% of the samples and 38% 

 of the specimens, while clay substrates contained 32% of the 

 samples and 32% of the specimens; silt sediments contained 

 11% of the samples and 15% of the specimens, while sand 

 contained 28% of the samples and 12% of the specimens. 

 Gravel, sand-gravel, till, shell, and sand-shell substrates con- 

 tained between 1 and 4% of the samples and from 1.3 to 0.1% 

 of the specimens (Table 151). 



Thyasira flexuosa forma gouldii Philippi 1845. Flexuose cleft 

 clam. Figure 112. 



The flexuose cleft clam occurs in both the North Atlantic 

 and North Pacific Oceans and is fairly widely distributed 

 throughout Arctic regions. In the Atlantic it ranges from 

 Greenland and Labrador to Cape Hatteras, N.C. ; in the North 

 Pacific it ranges from the Bering Sea to offSan Diego, Calif. ; it 

 is also reported from northern Eurasian waters and in north- 

 ern European waters (Johnson 1934; La Rocque 1953; Ockel- 

 mann 1958; Clarke 1962; Morris 1973; Abbott 1974). 



This tiny common bivalve is represented in the NEFC 

 collection by 415 specimens from 37 samples (Table 5). 



Our samples are from the Gulf of Maine region, the Cape 

 Cod region, and the periphery and the outer aspects of 

 Georges Bank onto the Mid-Atlantic continental shelf, with 

 two samples in slope water off the mouth of Chesapeake Bay 

 (Fig. 112; Theroux and Wigley footnote 4, table 201). 



Ockelmann (1958) considered this to be a panarctic species 

 and stated that it probably is continuously circumpolar, while 

 Coomans (1962) placed it in the Arctic, Boreal, Virginian, and 

 Carolinian provinces in the northwest Atlantic and Celtic in 

 European waters; Gosner (1971) placed it in the Boreal pro- 

 vince. 



The reported depth range for this species is from 2 to 2,685 

 m (Clark 1962). 



Our samples are from water depths which range between 32 

 and 720 m with a mean of 155 m. In terms of depth range 

 groupings, this species seems to prefer the mid-continental 



shelf depth grouping, 50-99 m, in which 57% of the samples 

 and 55% of the specimens are grouped; the 100-199 m group- 

 ing contains 22% of the samples and 5% of the specimens; the 

 200-499 m and the 500-999 m groupings each contain 8% of the 

 samples but 4 and 27%, respectively, of the specimens; the 

 only other range grouping in which specimens of this species 

 are grouped is the 25-49 m grouping which contains 5% of the 

 samples and 9% of the specimens (Table 152). 



As with other members of this genus the medium to fine 

 substrates appear to be preferred above coarser ones. The 

 majority of both samples and specimens occurred in silty sand 

 substrates which contained 38% of the samples and 45% of the 

 specimens; sand sediments contained 33% of the samples and 

 1 1% of the specimens, while clay substrates contained 22% of 

 the samples and 34% of the specimens; silt sediments con- 

 tained 5% of the samples and 10% of the specimens, while 

 sand-gravel, the only coarse grained substrate in which mem- 

 bers of this genus were found, contained 3% of the samples 

 and 0.5% of the specimens (Table 153). 



Thyasira pygmaea Verrill and Bush 1898. Figure 113. 



Johnson (1934), La Rocque (1953), and Abbott (1974) re- 

 port this species as occurring from Halifax, Nova Scotia, to 

 Martha's Vineyard, Mass. 



There are 64 specimens from 8 samples of this small bivalve 

 species in the NEFC collection (Table 5). 



Our samples are from the continental slope southeast of 

 Nova Scotia, in the Gulf of Maine, the outer aspects of the 

 Georges Bank continental slope, and two samples on the 

 southern New England continental shelf south of Nantucket 

 Shoals (Fig. 1 13: Theroux and Wigley footnote 4, table 202). 



The depth range for this species, according to the above- 

 cited authors, is 377 to 913 m. 



Our samples are from depths which range between 62 and 

 720 m with a mean of 308 m. The majority of both samples and 

 specimens are in the 200-499 m grouping which contains 38% 

 of the samples and 45% of the specimens; the 50-99 m and the 

 500-999 m groupings each contain 25% of the samples and 28 

 and 25% of the specimens, respectively; smallest amounts are 

 in the 100-199 m grouping which contains 13% of the samples 

 and 2% of the specimens (Table 154). 



Thyasira pygmaea was found in three sediment types with 

 the majority of both samples and specimens occurring in clay 

 substrates which contained 50% of the samples and 56%> of the 

 specimens; silty sand substrates yielded 38% for samples and 

 41% of the specimens; sand substrates accounted for 13% of 

 the samples and 3% of the specimens (Table 155). 



Thyasira subovata Jeffreys 1881. Figure 113. 



Both Johnson ( 1934) and Abbott ( 1974) listed this species as 

 occurring off Martha's Vineyard, Mass., while Clarke (1962) 

 reported that it occurs in North American waters, western 

 European waters, and at the Canary Islands. 



The NEFC collection contains 18 specimens from 7 sam- 

 ples of this uncommon species (Table 5). 



Our samples are from the Southern New England continen- 

 tal shelf and slope region south of Nantucket Shoals (Fig. 1 13; 

 Theroux and Wigley footnote 4, table 203). 



Both Johnson and Abbott reported a 915 m depth for this 

 species, while Clarke ( 1962) listed it as occurring between 300 

 to 2.564 m. 



28 



