2 8 2 DISCOVERY REPORTS 



Trigonium arcticum (Brightwell) Cleve. (PI. X, fig. i.) 



Cleve, 1868, vol. xxiv, p. 663. 



Mann, 1907, vol. x, p. 290. 



Triceratium arcticum Brightwell, 1853, p. 250. 



Biddulphia arctica (Brightwell) Boyer, 1901, p. 714. 



Cells triangular in valve view, somewhat rectangular in girdle view. Valves triangular, 

 sides straight, slightly convex or weakly concave. Angles rounded, not produced. 

 Valve surface slightly raised or inflated in the central area, curving weakly down to the 

 edge of the valve, where it falls sharply to the valve margin. Valve mantle straight, 

 rather short. Valves covered with polygonal areolation. Areoles small at the centre, 

 slightly increasing in size at half the radius, after which they decrease again. Areoles 

 arranged in curved radiating lines, often dichotomously branched. Areolation entire 

 over the whole surface. Areoles furnished with secondary structure upon the lower wall 

 in the form of small poroids, open upon the inner side. Angles of valve furnished with 

 bosses of fine pores, shaped somewhat like a thumb-mark. Bosses not produced, often 

 lower than the central area of the valve surface. Valve mantle bearing no constriction. 

 Girdle usually short, covered with fine puncta arranged in straight lines in the pervalvar 

 axis. Chromatophores : numerous rounded bodies. Diameter of valve 260-340^, 

 mostly 320,0. ; pervalvar axis 200^. 



This species is subject to great variation in size and outline. The bosses of fine pores 

 in the angles of the valves exude short mucous stipes or cushions, by which the cell 

 attaches itself to the substratum. A littoral diatom, not a true member of the plankton 

 and commonly found epiphytic upon larger algae. It has been reported from most 

 northern European coasts. The specimens obtained at St. WS 481 were unusually 

 large, and showed little variation in size or shape. 



Observed at Sts. WS 481, 622, 623. 



Genus Pseudo-triceratium Grunow 

 Grunow, 1884 



Pseudo-triceratium cinnamomeum (Greville) Grunow. 



Grunow, 1884, p. 83. 



Triceratium cinnamomeum Greville, 1863, p. 232. 



Valves triangular, sides straight or nearly so, with broadly rounded angles. Valve sur- 

 face covered with fine punctation, somewhat irregularly arranged in the central area, 

 but subradial towards the margin. The margin of the valve is furnished with several 

 small but stout spinulae. The lines of puncta which radiate towards the spinulae are 

 more prominent than the remainder. Diameter of cell 36/u.. 



Observed at St. 434. 



