SYSTEMATIC ACCOUNT 239 



A neritic species, fairly common in temperate waters particularly in the northern 

 hemisphere. Recorded from the Brazilian coast and from the Peru Current off the 

 Chilean coast. This is the first record of this species in the Pacific Ocean. 



Observed at Sts. 722, 723; WS 593, 647, 648, 649, 650. 



Thalassiosira gravida Cleve. (PL XI, fig. 10.) 



Cleve, 1896, vol. xxn, p. 12, pi. 2, figs. 14-16. 

 Hustedt, 1928, p. 325, fig. 161. 

 Gran, 1905, p. 18, fig. 12. 

 Lebour, 1930, p. 59, fig. 31. 



Cells discoid, rectangular in girdle view. Valves circular, united to form chains by a 

 thick mucous thread exuded from a central pore ; thread short, but often undulating. 

 Valves flat or nearly so, edges slightly rounded. Structure of the valve very faint, com- 

 posed of radial striae, moniliform. Valves furnished with small spinulae irregularly dis- 

 posed, and one fairly stout apicule, usually marginal. Resting spores were frequently 

 observed, and are characterized by their strongly convex valves, convexity unequal, and 

 large areolate structure. Chromatophores : ten to eighteen small plates, usually adhering 

 to the valves. Diameter of valve 20-58/*, mostly 50^ ; pervalvar axis 8-25/x ; intercellular 

 thread 30-40/x. 



Distribution, temperate to subpolar, common in neritic plankton in all northern seas. 

 Observed fairly frequently in plankton around South Georgia. 



Observed at Sts. 508, 509, 510, 511, 512, 513. 



Thalassiosira hyalina (Grunow) Gran. 



Gran, 1897, p. 16, pi. 1, figs. 17, 18. 



Gran, 1905, p. 17, fig. 11. 



Hustedt, 1928, p. 323, fig. 159. 



Lebour, 1930, p. 61, fig. 32. 



Coscinodiscus hyalinus Grunow ex Cleve et Grunow, 1880, p. 113. 



Cells discoid, narrowly rectangular. Valves circular, united to form chains by means 

 of central mucous thread. Distances between frustules often unequal. Valves flat, with 

 slightly rounded or bevelled edges. Valves furnished with fine moniliform striation ; 

 striae radial, structure best seen when the valves are mounted dry. A single row of 

 small spinulae surround the margin of the valve, including one larger apicule. Chro- 

 matophores: usually eight to ten small plates. Diameter of valve 24-481*, mostly 40/* ; 

 pervalvar axis about 10/x ; intercellular thread 6-io/x and 40-50/x. 



A neritic species which has been recorded only from northern waters, its occurrence 

 therefore off the coast of South Africa is somewhat unusual, it had probably been 

 carried north-eastwards from the neighbourhood of Bouvet Island. 



Observed at Sts. 260, 262. 



Thalassiosira subtilis (Ostenfeld) Gran. 



Gran, 1900^, p. 117. 

 Ostenfeld, 1903, p. 563, fig. 119. 



