SYSTEMATIC ACCOUNT 293 



Chaetoceros coarctatum Lauder. (PI. VI, figs. 7, 8.) 



Lauder, 1864, p. 79, pi. 8, fig. 8. 

 Gran, 1905, p. 68, fig. 80. 

 Karsten, 1905, p. 120, pi. 16, fig. 6. 

 Hustedt, 1930, p. 655, fig. 370. 

 Lebour, 1930, p. 119, fig. 85. 

 Chaetoceros rudis Cleve, 1901, p. 308. 



Cells united to form short chains of usually twelve to sixteen cells. Valves elliptical 

 to circular, strongly siliceous. Valves adpressed, no distinct foramina present. Valve 

 mantle usually deep, often a little deeper than the connective zone. Bristles coarse, 

 emanating from the margin of the valve. Bristles proceeding outwards in different 

 planes, almost perpendicular to the axis of the chain for a short distance and then 

 strongly curved downwards, often recurved, crossing the pervalvar axis. Bristles usually 

 thickest at about half their length, often bearing faint ribs and strong spines throughout 

 their entire length. Terminal bristles usually very strong and convergent. Chromato- 

 phores: numerous cocciform bodies. Diameter of valve 30-40/x. 



An oceanic species having a wide distribution in tropical and subtropical seas. It was 

 observed frequently around the coast of Africa, particularly on the Indian Ocean side, 

 and in the Peru Current material from the Pacific. 



This species is very liable to parasitism, and it is rather remarkable to note that 

 although the specimens examined were obtained from areas widely separated, every one 

 without exception was heavily infested by Vorticella oceanica Zach. This is probably a 

 case of symbiosis. The diatom cells were very healthy and the cell contents did not 

 appear to have suffered as a result of the association. 



Observed at Sts. 293, 670, 681, 1373, 1584; WS 709, 710, 714. 



Chaetoceros convolutum Castracane. 



Castracane, 1886, p. 78. 



Gran, 1905, p. 69, fig. 82. 



Meunier, 1910, p. 218, pi. 24, figs. 17-19. 



Hustedt, 1930, p. 668, fig. 378. 



Lebour, 1930, p. 119, fig. 86. 



Cells united to form chains, usually curved or slightly twisted. Valves dissimilar, 

 almost circular; the upper rounded, the lower fiat. Valve mantle almost as deep as the 

 girdle and clearly defined by a small sharp constriction at the point of juncture. Bristles 

 long, thin, but strong. Those of the upper valve arise near the centre, those of the lower 

 valve emanate from near the valve margin, and appear to lie closer to the axis of the chain 

 than do those of the upper valve. Bristles armed with short but distinct spines throughout 

 their entire length. All bristles are bent towards the lower end of the chain. The bristles 

 of the lower valve of the terminal cell often converge and cross. Foramina small, 

 frequently obscure. Chromatophores : numerous small rounded bodies, nucleus central. 

 Diameter of valve 15-30^. 



This species is very common in the Atlantic and northern European seas. 



Observed at Sts. 440, 690; WS 100. 



