MARINE MOLLUSKS — MACGINITIE 159 



is little evidence of this projection in specimens 8 mm. and less in 

 length (pi. 4, fig. 11; pi. 21, fig. 4). 



A specimen with a shell measuring 12.7 by 6.9 by 6 mm. that was 

 taken at 184 feet on Aug, 30, 1949, was entirely enclosed within a 

 byssal capsule, and within this capsule were a large number of em- 

 bryonic clams still within their egg membranes but with definite valves 

 developed. The egg membrane was from 20 to 55/x longer than the 

 embryonic clams. Two of the embryos measured 389m (434^1 with 

 the egg membrane), 5 measured 434)ti (1 was 454iu long with the egg 

 membrane), and 1 measured 462/z. 



A specimen 13 mm. long is at least 3 years old. 



Distribution: Circumboreal. Arctic Ocean to Puget Sound (Dall, 

 1921); Jan Mayen (?), western Greenland (few), Iceland (?), Spitz- 

 bergen, Finmarken, Polar Sea of Siberia (Jensen, 1912); Greenland 

 to North Carolina (Johnson, 1934). 



Musculus discors (Linnaeus, 1767) 

 Var. laevigatus and forma substriatus Gray 



Plate 18, figure 10; Plate 21, figure 5 



Mytilus discors Linnaeus, 1767, ed. 12, p. 1159. 



Modiola laevigata Gray, 1824, p. 244. 



Modiola laevigata var. substriata Gray, 1824, p. 245. 



Modiolaria laevigata G. Sars, 1878, p. 29, pi. 3, figs. 3a-b (good illustrations). 



Modiolaria discors Jensen, 1912, p. 57, pi. 3, figs. 6a-b. 



Modiolaria discors var. laevigata Jensen, 1912, p. 57, pi. 3, figs. 4a-b. 



Modiolaria discors var. substriata Jensen, 1912, p. 58, pi. 3, figs. 5a-c. 



Musculus discors Abbott, 1954, p. 355, pi. 28e. 



Musculus laevigatus Abbott, 1954, p. 355, pi. 28f. 



Varieties of this species, especially individuals up to 9 mm. in 

 length, were fairly abundant in the deeper stations. The largest in- 

 dividual, 29 mm. long, came from 741 feet; 1 (21 mm.) came from 477 

 feet; 1 (14 mm.) from 453 feet; 2 (20 and 24.5 mm.) from 438 feet; 

 1 (24.6 by 16.1 by 9.4 mm.) from 175 feet; 1 (13 mm.) from 122 feet; 

 1 (28.9 mm.) from Eluitkak Pass on Aug. 1, 1950; 11 (from 7.6 to 

 11.2 mm.) from Eluitkak Pass on Aug. 30, 1948; and 3 (11.5, 16.5, 

 and 23 mm.) from Eluitkak Pass on Aug. 6, 1948. 



Smaller individuals were dredged as follows: 4 (up to 5 mm.) at 130 

 feet on Aug. 9, 1949; 1 (about 3 mm.) at 152 feet; 5 (up to 11 mm.) 

 at 175 feet; 3 (up to 6 mm.) at 184 feet; 1 (3 mm.) at 420 feet; about 

 30 (up to 9 mm.) at 438 feet; 8 (up to 13.5 mm.) at 453 feet; about 100 

 (from 2 to 13.2 mm.) at 477 feet; and 18 (from 3 to 15.5 mm.) at 

 741 feet. 



Juveniles up to 9 mm. in length were frequently found attached to 

 erect colonies of bryozoans. For example, 3 specimens (up to 6 mm. 

 long) were found attached to Tricellaria erecta at 184 feet; 1 (10 mm.) 



