The occurrence of samples containing bivalves was highest 

 in sand and silty sand substrates, 32 and 13%, respectively. 

 Next highest density of samples occurred in the finest grained 

 substrates, silt (8%) and clay (7%). Areas of coarser grained 

 sediments, gravel, shell, and sand-shell, each contained < 8% 

 of the total number of samples, while sand-gravel sediments 

 contained < 0.5% of the samples. Twenty-one percent of the 

 samples are unclassified with regard to sediment type. 



Specimen density in the various sediment types very close- 

 ly approximates that of sampling intensity. Greatest numbers 

 of organisms occurred in the sandy and muddy substrates 

 with fewer in the coarse textured sediments. Table 4 lists the 

 occurrence of bivalve samples and specimens in relation to 

 bottom sediments. 



The occurrence frequency of individual bivalve taxa adjust- 

 ed for distribution of sampling intensity among the various 

 depth range groupings or sediment types, although not tabu- 

 lated herein, may be calculated from the data contained in 

 Tables 3 and 4 for total samples, and Tables 6 through 327 for 

 individual taxa. 



SYSTEMATIC ARRANGEMENT 



The systematic arrangement of R. T. Abbott (1974) has 

 been, for the most part, followed in this report. The only 

 exceptions involve the placement of families in the orders 

 Veneroida and Pholadomyoida where we have followed the 

 arrangement of N. D. Newell (in R. C. Moore, 1969a, vol. 1, 

 p. N218). 



There are 36 samples containing 76 specimens in our collec- 

 tion which we were only able to classify as Bivalvia. A variety 

 of reasons necessitated this classification; the most common 

 cause for failure to arrive at a lower taxonomic designation 

 was lack of shell, especially of the smaller, thinner shelled 

 species, whose hard parts were dissolved away by preserva- 

 tives which were too acid. Another was shell structure so 

 badly damaged that no definite determination, other than that 

 of Bivalvia, could be made through examination of soft parts. 



Further detailed discussion concerning this category would 

 be pointless except to direct the interested reader's attention 

 to the accompanying figures and tables which provide in- 

 formation about distributional and environmental parameters 

 relating to areas where unidentifiable bivalve material was 

 encountered. (See Fig. 21, and Tables 5, 6, 7; and Theroux 

 and Wigley footnote 4, table 32.) 



DISTRIBUTIONAL AND ECOLOGICAL 

 DISCUSSION 



Class BIVALVIA 



Subclass PALAEOTOXODONTA 



Order NUCULOIDA 



The NEFC Specimen Reference Collection contains two 

 specimens from two samples of organisms which are classi- 

 fied to the order level Nuculoida (Table 5). 



One of the samples is from east of Cape Cod and the other 

 sample is from Vineyard Sound, Mass. (Fig. 79; Theroux and 

 Wigley footnote 4, table 134). 



The depth of our samples is 15 and 103 m, placing them in 

 two separate depth range groupings, each of which contained 

 50% of samples and specimens; the groupings concerned are 

 the 0-24 m and the 200-499 m. 



Only one sample contained information relating to bottom 

 sediments; this sample was obtained from a clay substratum. 



Family NUCULIDAE 

 Genus Nucula Lamarck 1799 



Nucula delphinodonta Mighels and Adams 1842. Delphinula nut 

 clam. Figure 71. 



The distribution of the delphinula nut clam extends from 

 Labrador to Maryland on the east coast of the United States 

 as well as being moderately well distributed in Arctic regions 

 and in northern Europe (Johnson 1934; La Rocque 1953; 

 Ockelmann 1958; Clarke 1962; Abbott 1974). 



This tiny bivalve is represented in our collection by 2.092 

 specimens from 145 samples (Table 5). 



The NEFC samples range from the Scotian Shelf through 

 the Gulf of Maine-Georges Bank complex onto the Southern 

 New England shelf and the Mid-Atlantic Bight region south to 

 the offing of Chesapeake Bay (Fig. 71; Theroux and Wigley 

 footnote 4, table 123). 



This species occupies the Boreal and Virginian provinces in 

 eastern North America (Coomans 1962): Gosner (1971) 

 placed it in the Boreal province. 



The delphinula nut clam enjoys a fairly wide bathymetric 

 range, occupying depths between 17 and 2.361 m (Clarke 

 1962). 



The range in depth occupied by the samples in the NEFC 

 collection is from 11 to 1,894 m with a mean of 197 m. The 

 majority of both samples and specimens. 41 and 78%. respec- 

 tively, are in the 50-99 m depth range grouping: considerably- 

 smaller amounts occur in other depth range groupings: abun- 

 dance with increasing depth range is as follows: the 0-24 m 

 depth range grouping contains 6% of the samples and 2% of 

 the specimens; the 25-49 m grouping. 14% of the samples and 

 11% of the specimens; the 100-199 m grouping. 18 and 4%, 

 respectively; the 200-499 m grouping, 14 and 2%. respective- 

 ly; the 500-999 m grouping, 5 and 1%, respectively; and the 

 1.000-1,999 m depth range grouping contains 4% of the sam- 

 ples and 2% of the specimens (Table 8). 



This bivalve was found in all of the sediment types consid- 

 ered in this report. Abundance with decreasing particle size is 

 as follows: gravel contained 3% of the samples and 0.7% of 

 the specimens: sand-gravel. 4 and 0.4%. respectively; till 

 sediments, 4% of the samples and 0.8% of the specimens: 

 shell contained 2% of the samples and 0.5% of the specimens: 

 sand-shell contained 0.7% of the samples and 0.2% of the 

 specimens; sand substrates, 24 and 16%, respectively: silty 

 sand substrates contained the highest amounts. 29% for sam- 

 ples and 72% for specimens; silt contained 13% of the samples 

 and 3% of the specimens ; and clay substrates 20% for samples 

 and 6% for specimens (Table 9). There are two samples con- 

 taining six specimens which are unclassified with regard to 

 sediment type. 



Nucula proximo Say 1822. Atlantic nut clam. Figure 72. 



The range of the Atlantic nut clam is from Newfoundland to 

 Florida and Texas and it also occurs at Bermuda (Johnson 

 1934; Morris 1951 and 1973: La Rocque 1953: Ockelmann 

 1958; Abbott 1968, 1974; Emerson et al. 1976). 



This tiny bivalve is a common member of the Northeast 

 U.S. bivalve fauna. It bears the honor of beine the most 



