29 6 DISCOVERY REPORTS 



spines throughout their entire length. The bristles upon the lower valve of a terminal 

 cell are directed parallel with the axis of the chain, but are directed obliquely if the cell 

 is solitary. Chromatophores : several small rounded bodies. Diameter of valve 16-50/*. 



A characteristic Antarctic diatom, found in enormous quantities around South 

 Georgia, in the Weddell Sea, the Bellingshausen Sea, and the Ross Sea. This species, 

 together with Corethron criophilum Castracane, often amounts to over 90 per cent of 

 the total phytoplankton in some areas in the extreme south. 



Observed at Sts. 301, 368, 369, 378, 379, 380, 381, 382, 383, 384, 386, 388, 425, 432, 

 460, 461, 463, 475, 477, 478, 479, 480, 481, 482, 501-513, 551, 570, 574, 575, 576, 577, 

 578, 580, 615, 619, 626, 659, 661, 663, 664, 666, 1356, 1358, 1359, 1362; WS 101, 104, 

 474, 540, 541, 545, 547, 548, 549, 550, 551, 55 2A, 569, 571, 580, 593, 594, 598, 600, 

 602; RS 9, 17, 27; MS 86, 88, 94, 99, 100, 103. 



Chaetoceros peruvianum Brightwell. (PI. XIII, fig. 6.) 



Brightwell, 1856, p. 107, pi. 7, figs. 16-18. 



Gran, 1905, p. 70, fig. 84. 



Karsten, 1906, p. 166, pi. 31, fig. 4. 



Hustedt, 1930, p. 671, fig. 380. 



Chaetoceros peruvio-atlanticum Karsten, 1907, p. 385, pi. 43, fig. 1. 



Cells sometimes united to form short chains, but mostly solitary. Valves elliptical, 

 dissimilar, the upper slightly convex, the lower flattened. Valve mantles deep, con- 

 stricted sharply as they reach the girdle ; girdle narrow. The bristles are long and stiff. 

 Those of the upper valve emerge more or less from the centre, and occupy most of the 

 valve surface, abutting each other at the point of emergence, reducing the actual central 

 area of the valve to a small globiform opening. These upper bristles continue somewhat 

 obliquely in long sweeping curves. The bristles of the lower valve emerge obliquely and 

 continue more or less in a direction parallel with the pervalvar axis of the cell or the axis 

 of the chain. There is a small spine upon the valve surface of the lower valve, occupying 

 a median position. The bristles are striate, and armed with small spines. Chromato- 

 phores: several rounded bodies. Diameter of valve, polar axis 10-4411. 



A very variable oceanic species, widely distributed throughout temperate and 

 tropical seas. It was observed frequently around the coast of South Africa, off the Falk- 

 land Islands, and in great numbers in the Pacific material from the Humboldt Current. 



A number of forms and varieties have been established based upon the depth of the 

 valve mantle, and the degree of obliquity and thickness of the bristles. Owing to the 

 variability of the characters upon which these forms and varieties are based I find that 

 these subspecific ranks served no useful purpose, as the enormous amount of material 

 at my disposal provided every intermediate gradation. 



Observed at Sts. 260, 425, 427, 428, 432, 433, 434, 435, 437, 438, 439, 440, 450, 453, 

 664; WS 100, 593, 594, 598, 600, 601, 666, 703, 709, 715, 716. 



Chaetoceros seychellarum Karsten. 

 Karsten, 1907, p. 387, pi. 43, fig. 4. 



