contains 75% of the samples and 75% of the specimens, and 

 the 200-499 m grouping contains 25% of each (Table 300). 



Fifty percent of both samples and specimens occurred in 

 two substrate types: sand and silty sand (Table 301). 



Family THRACHDAE 



Our collection contains 36 specimens from 19 samples 

 which are classified at the familial level Thraciidae (Table 5). 



Samples containing members of this family are from the 

 inner and outer continental shelf ranging between the mouth 

 of Delaware Bay south to slightly north of Jacksonville, Fla. 

 (Fig. 107; Theroux and Wigley footnote 4, table 194). 



Our samples are from depths which range between 13 and 

 365 m with a mean of 68 m. The 0-24 m depth range grouping 

 contains 37% of the samples and 33% of the specimens; the 

 25-49 m grouping contains 26% of the samples and 25% of the 

 specimens; the 50-99 m grouping contains 21% of the samples 

 and 25% of the specimens; the 100-199 m grouping is one 

 exception with low quantities, containing 5% of the samples 

 and 3% of the specimens, while the 200-499 m grouping con- 

 tains 11% of the samples and 14% of the specimens (Table 

 302). 



Three sediment types yielded samples which contained 

 members of this family with sand containing the majority, 

 74% of samples and 78% of the specimens; sand-shell sub- 

 strates contained 16% of samples and 8% of the specimens, 

 while silty sand substrates contained 11% of the samples and 

 14% of the specimens (Table 303). 



Genus Thracia Leach 1823 



Thracia conradi Couthouy 1839. "Conrad's thracia. Figure 106. 



The geographic range of this species is from the Canadian 

 Maritime Provinces to North Carolina (Johnson 1934; La 

 Rocque 1953; Ockelmann 1958; Abbott 1968, 1974; Morris 

 1973). 



Thracia conradi is fairly common and oftentimes frequent- 

 ly encountered; the NEFC collection contains 10 specimens 

 from 6 samples of this species (Table 5). 



Our samples are distributed from slightly south of Grand 

 Manan Island off the coast of Maine to slightly north and east 

 of the mouth of Delaware Bay (Fig. 106; Theroux and Wigley 

 footnote 4, table 191). 



This species occupies the Boreal and Virginian provinces 

 (Coomans 1962); Dance (1974) placed it in the Boreal and 

 Transatlantic provinces. 



Conrad's thracia ranges from just below the low water mark 

 to approximately 275 m (Abbott 1968, 1974; Morris 1973). 



Our samples are from water depths which range between 34 

 and 126 m with a mean of 70 m. The majority of both samples 

 and specimens are in the 0-24 m depth range grouping, which 

 contains 50% of the samples and 70% of the specimens, while 

 the 50-99 m grouping contains 33% of the samples and 20% of 

 the specimens; smallest amounts are in the 25-49 m depth 

 range grouping which contains 17% of the samples and 10% of 

 the specimens (Table 304). 



l2 Abbott f!974) has "Couthouy. 1838" for this species, it shold be Couthouy 

 1839, see under References. 



Specimens of T. conradi were found in four sediment types ; 

 33% of the samples occurred in both silty sand and silt and 

 17% of the samples occurred in till and sand. The distribution 

 of specimens, however, differed in each sediment type with 

 silt containing 40%, till, 30%, silty sand, 20%, and sand, 10% 

 of the specimens (Table 305). 



Thracia myopsis Moller 1842. Figure 107. 



This species, which is widely distributed throughout Arctic 

 regions ranges south to the Massachusetts coast (Johnson 

 1934; La Rocque 1953; Ockelmann 1958; Abbott 1974). 



This uncommon bivalve is represented in the NEFC collec- 

 tion by six specimens from three samples (Table 5). 



One of the three samples in the NEFC collection was from 

 Browns Bank, another from east of Cape Ann, Mass., and the 

 third at the tip of Cape Cod, Mass. (Fig. 107; Theroux and 

 Wigley footnote 4, table 192). 



Ockelmann (1958) reported that this species is panarctic in 

 the American sector only. 



The depth range for this species according to Johnson 

 (1934), Ockelmann (1958), and Abbott (1974), is between 2 

 and 350 m. 



Our samples are from water depths which range between 95 

 and 1 14 m with a mean of 105 m. The 50-99 m and 100-199 m 

 depth range groupings each contain 50% of the samples while 

 the former contains 60% and the latter 40% of the specimens. 

 There is one sample containing one specimen which does not 

 have any depth information in the sampling data. 



Two of the samples containing five specimens were found 

 in gravel substrates. One sample containing one specimen is 

 unclassified with regard to sediment type. 



Thracia septentrionalis Jeffreys 1872. Northern thracia. Figure 

 107. 



The northern thracia is widely distributed throughout Arc- 

 tic regions and ranges from Greenland south to Block Island. 

 R.I. (Johnson 1934; La Rocque 1953; Ockelmann 1958; Mor- 

 ris 1973; Abbott 1974). 



Thracia septentrionalis is represented in the NEFC collec- 

 tion by 46 specimens from 13 samples (Table 5). 



Our samples are from the Georges Bank region and the 

 surrounding offshore waters of Cape Cod, Mass. (Fig. 107; 

 Theroux and Wigley footnote 4, table 193). 



Gosner (1971) placed this species in the Boreal zoogeo- 

 graphic province and Ockelmann (1958) reported that it is 

 panarctic in the North Atlantic. 



The northern thracia occupies water depths which range 

 between 9 and 113 m (Ockelmann 1958). 



Our samples are from depths which range between 23 and 

 74 m with a mean of 54 m. In terms of depth range groupings 

 there is a diminution of both sample and specimen abundance 

 with decreasing depth range from 50-99m. The majority of 

 both samples and specimens are in the 50-99 m depth range 

 grouping which contains 62% of the samples and 74% of the 

 specimens, while the 25-49 m grouping contains 31% of the 

 former and 22% of the latter; the 0-24 m depth range grouping 

 contains the smallest amount of both samples and specimens 

 with 8% of the former and 4% of the latter (Table 306). 



Our samples of the northern thracia were found in two 

 sediment types with the majority of them occurring in sand 



55 



