34. Face of semicell with granular decorations but without a protuber- 

 ance or sweUing; spines at the angles horizontally extended and 

 usually divided (see Xanthidium armatum, however); cells narrowly 



oval in side view Spinocosmarium 



PI. 4 Fig. 3 

 34. Face of semicell with a swelling; cell appears broadly oval in side 

 view; with simple spines (but divided in Xanthidium 



armatum ) Xanthidium, 



PI. 3, Fig. 7, 12; PI. 4, Fig. 14 



II. DIATOMS 



The Diatoms ( Bacillariophyceae ) belong to the division (phy- 

 lum) of the plant kingdom known as the Chrysophyta. The 

 chromatophores contain a predominating brown pigment, diatomin 

 in addition to an abundance of xanthophyll and carotene, and 

 chlorophyll. Hence healthy Diatoms are golden-brown or yellow- 

 ish-green in color. Stored food accumulates as oil (in globules) 

 and the iodine test for starch is negative. The wall is sihceous 

 (glassy) and brittle and ordinarily is etched with lines, rows of dots 

 or puncta. One of the pecularities of the Diatoms is the gliding or 

 jerky movement often exhibited as a result of currents of water 

 and mucilage through canals in the wall and through pores to 

 the exterior. 



The cell, usually referred to as a frustule when the shell or wall 

 is under consideration, is varied in shape, as is also the pattern of 

 ornamentation. The wall is composed of two sections, one of 

 which is slightly larger, the epivalve. This overlies and overlaps 

 the edges of the smaller, hypovalve much as a lid fits over a box. 

 The parts are known as valves. 



There are two groups of Diatoms, although precise limitations 

 break down in some instances and some diatomists have come to 

 disregard the two groupings. The two Orders, however, provide 

 convenient assignment of two principal types of Diatoms. 1. The 

 Centrales have cells which in the main are radially symmetrical, 

 often round when seeii from the top or valve view; wall ornamenta- 

 tions are radially disposed. 2. The Pennales, on the other hand, 

 are elongate, cigar-shaped, boat-shaped, rectangular, or wedge- 

 shaped, and the wall decorations are bilaterally symmetrical. The 

 cells are often rectangular when seen from the side, or in girdle 

 view. 



In the Pennales many of the genera have a distinct median or 

 marginal canal in the wall of one or both valves, extending parallel 

 with the long axis. Within the canal there is a raphe which shows 

 as a straight or curved line, usually seen in valve view. Sometimes 

 a narrow, clear area appears in the midline of the valve where 

 there are no wall markings. This region which appears as a line 



[939] 



