3 io DISCOVERY REPORTS 



wall and upon lines of plasma which radiate from the nucleus. Auxospores and resting 

 spores have been observed ; the former occur in some species as a lateral projection at 

 right angles to the pervalvar axis of the parent cell. Some species, particularly those of 

 the R. styliformis group, are liable to infection by parasites. The alga Richelia inter- 

 cellularis was observed, but not frequently, within the cells of R. styliformis from tropical 

 stations upon the Indian Ocean side of Africa. Several species exhibit what I describe 

 as polyphasic tendencies, that is several species are found possessing more than one set 

 of morphological characters, and in some specimens both sets of characters are present, 

 indicating specific unity of the variants. The chief of these are R. alata and R. hebetata. 

 All of the so-called varieties and forms of these species I have reduced to synonyms and 

 have described them as phases of a plastic species system. 



Rhizosolenia alata Brightwell. 



Brightwell, 1858, p. 96, pi. 5, fig. 8. 



Gran, 1905, p. 56, fig. 68. 



Hustedt, 1929, p. 600, fig. 344. 



Lebour, 1930, p. 88, fig. 58. 

 Cells tubular, elongated, cylindrical, straight. Valves shortly conical, attenuated to 

 produce an eccentric process. Process truncate or slightly rounded, almost parallel with 

 the main axis of the cell or very slightly incurved. Valve furnished at the base with a faint 

 depression which corresponds to the apex of the neighbouring cell, and fits into it when 

 in chain formation. Terminal spine absent. The connective zone is composed of two 

 rows of dorsi-ventral scale-like intercalary markings, which appear as a zigzag line in 

 lateral view. Scales rhombic, furnished with very fine striation. Chromatophores : 

 numerous cocciform bodies, nucleus almost central. Diameter of cell 10-20^ ; pervalvar 

 axis up to 700/x. 



An oceanic species having a wide distribution throughout temperate and subtropical 

 seas. It was one of the most common diatoms observed in the ' Discovery ' material, and 

 was noticed in practically all samples except those from off the Brazil coast ; sometimes 

 it occurred in very great numbers. 



Observed at Sts. 263, 300-305, 335, 336, 337, 338, 339, 340, 365, 368, 369, 378, 379, 

 380-388, 425, 427, 428, 431-436, 438, 439, 440, 450, 460, 475, 478, 479, 505-508; 511, 

 513, 575-578, 580, 615, 659, 666, 670, 671, 675, 681, 684, 687, 690, 1356, 1358, 1359, 

 1362, 1369, 1373, 1584, 1586, 1589; WS469, 540, 541, 545, 547, 549, 550, 551, 552A, 

 598, 703, 705"7 I 2, 7H- 



"gracillima" phase. 



Rhizosolenia alata, forma gracillima (Cleve) Grunow. 



Grunow, in Van Heurck, 1880-85, P 1 - 79> %• 8 - 



Hustedt, 1929, p. 601, fig. 345. 



Lebour, 1930, p. 90, fig. 59. 

 Very similar to the type structurally, but considerably thinner. The valve is somewhat 

 less conical than the type, but a little more produced. Diameter of valve 4-7/i ; pervalvar 

 axis up to 500 jtx. 



