Our samples are from depths which range from 62 to 567 m 

 with a mean of 256 m. Fifty-seven percent of the samples and 

 44% of the specimens are in the 50-99 m grouping. 29% of the 

 samples and 50% of the specimens in the 200-499 m grouping, 

 and 14% of the samples and 6% of the specimens are in the 

 500-999 m grouping (Table 156). 



Thyasira subovata specimens were found in four sediment 

 types with 43% of the samples and 56% of the specimens 

 occurring in silt substrates, and 29% of the samples and 28% 

 of the specimens occurring in sand substrates; both silty sand 

 and clay substrates each contained 14% of the samples, but 1 1 

 and 6%. respectively, for specimens (Table 157). 



Thyasira trisinuata Orbigny 1842. Atlantic cleft clam. Figure 

 114. 



It occurs in both the North Atlantic and North Pacific 

 Oceans and ranges from Labrador and Nova Scotia to the 

 southern half of Florida, and on into the West Indies in the 

 Atlantic: in the Pacific it ranges from Alaska to San Diego, 

 Calif. An interesting observation is that at least for the north- 

 west Atlantic there appears to be an increase in depth occurr- 

 ence with decreasing latitude (Johnson 1934; LaRocque 1953: 

 Ockelmann 1958: Clarke 1962; Abbott 1968, 1974; Morris 

 1973). 



Thyasira trisinuata is moderately common: the NEFC col- 

 lection contains 1,079 specimens of this species from 133 

 samples (Table 5). 



The NEFC samples range from the environs of the Nova 

 Scotian shelf, in the inshore regions of the Gulf of Maine, onto 

 the Southern New England shelf and south to Miami, Fla., 

 with, as mentioned earlier, increasing depth occurrence with 

 decreasing latitude (Fig. 1 14; Theroux and Wigley footnote 4, 

 table 204). 



Coomans (1962) placed the Atlantic cleft clam in the 

 Boreal, Virginian. Carolinian, and Caribbean zoogeographic 

 provinces while Gosner (1971) placed it in the Boreal and 

 Virginian provinces. 



The reported depth range for this species is from 22 to 2,361 

 m with the possibility of abyssal records having been trans- 

 ported from shallow waters (Clarke 1962). Other authors 

 report the deepest depth recorded as being 351 m (Johnson 

 1934; Abbott 1968, 1974). 



Our samples are from depths ranging between 23 and 2,520 

 m with a mean of 156 m. All of the depth range groupings in 

 this report contain specimens and samples of the Atlantic cleft 

 clam with the majority in the mid-continental shelf depth 

 range grouping of 50-99 m which contains 61% of the samples 

 and 62% of the specimens. The 100-199 m grouping contains 

 22% of the samples and 17% of the specimens, while the 

 200-499 m grouping contains 8% of the samples and 12% of the 

 specimens; other depth range groupings contain significantly 

 smaller amounts, the shallowest (0-24 m), and the two deepest 

 groupings ( 1 ,000-1 ,999 and 2,000-3,999 m) each contain < 1% 

 of both samples and specimens (Table 158). 



Specimens of the Atlantic cleft clam were found in all 

 sediment types, except shell; however, sand and silty sand 

 contained the majority of both samples and specimens, silty 

 sand was first with 40% of the samples and 53% of the speci- 

 mens while sand sediments contained 35% of the samples and 

 26% of the specimens; clay substrates contained 15% of the 



samples and 12% of the specimens, all other sediment types, 

 gravel, sand-gravel, till, and silt contained 5% or less of both 

 samples and specimens (Table 159). 



Thyasira sp. Figure 115. 



One of the reasons for the moderately large number of 

 specimens (734) and samples (142) of this genus in the NEFC 

 collection is the fact that this bivalve is tiny and has a very 

 soft, thin shell which is easily eroded in preservative. This 

 shell erosion results in only the soft parts of the organism 

 remaining in samples with subsequent difficulty in arriving at 

 a species determination (Table 5). 



Samples yielding specimens of the genus Thyasira in our 

 collection are from two major regions, one of which is the Gulf 

 of Maine proper, and another suite of samples from the outer 

 continental shelf and upper slope regions beginning on the 

 northeast peak of Georges Bank and extending south to 

 approximately Cape Hatteras, N.C. (Fig. 115; Theroux and 

 Wigley footnote 4, table 205). 



The depth range of samples containing Thyasira sp. is 15 to 

 2,645 m with a mean of 295 m. The mid to lower continental 

 shelf depth range groupings contain the majority of both 

 samples and specimens while groupings on either side of the 

 central portion contain significantly fewer samples and speci- 

 mens. The 100-199 m grouping contains 31% of the samples 

 and 31% of the specimens, while the 200-499 m grouping 

 contains 35% of the samples and 23% of the specimens; 11% 

 of the samples and 23% of the specimens are in the 500-999 m 

 grouping, and 13% of the samples and 18% of the specimens in 

 the 50-99 m grouping; the 0-24, 25-49, 1,000-1 ,999, and 2,000- 

 3,999 m groupings contain significantly smaller amounts of 

 both samples and specimens (Table 160). There is one sample 

 containing three specimens which does not contain any depth 

 information. 



The only sediment type devoid of specimens of this genus 

 was shell. The majority of both samples and specimens oc- 

 curred in the finer grained substrates with clay containing 

 34% of the samples and 33% of the specimens; silty sand 

 yielded 30% of the samples and 31% of the specimens; silt 

 contained 16% of the samples and 19% of the specimens, and 

 sand substrates, 11% of the samples and 11% of the speci- 

 mens. Considerably smaller amounts occurred in gravel. 

 sand-gravel, and till (Table 161). There are 8 samples contain- 

 ing 33 specimens which are unclassified with regard to sedi- 

 ment type. 



Family UNGULINIDAE 

 Genus Diplodonta Bronn. 1831 



Diplodonta sp. Figure 42. 



There are 90 specimens from 58 samples classified as Diplo- 

 donta sp. in the NEFC collection (Table 5). 



The distribution of samples containing members of this 

 taxon occurs in two groupings, one off Chesapeake Bay over 

 the edge of the continental shelf north of Cape Hatteras, 

 N.C, the other on the continental shelf ranging from south of 

 Cape Hatteras, N.C, to Miami, Fla. (Fig. 42; Theroux and 

 Wigley footnote 4, table 69). 



Our samples are from water depths ranging from 7 to 1 ,615 

 m with a mean of 63 m. Placement of samples in depth range 



29 



