80 ALG.E. 



and it illustrates well the process of division of the 

 frustules (fig. 10 /) . The valves are covered with 

 fine dots, and near each end of the frustules is 

 a projecting rim encircling it, and appearing as a 

 curved line extending beyond the margin of the 

 frustule in the front view. In Actinocy 1 clus un- 

 duldtus (PI. VI. fig. 5) the frustules are separate, 

 disk-shaped, and the valves are divided into six equal 

 parts by six rays, each alternate portion of the surface 

 of the valves being situated on a lower level than 

 those adjacent, so that an alteration in the focus is 

 required to bring into view the dots on any two ad- 

 jacent divisions of the valve. The surface of the 

 valves is covered with easily recognized dots. The 

 form of the surface is best seen in the front view 

 (fig. 5 /) when the frustule is placed on its edge. 



Rhabdonema arcudtum (PI. VI. fig. 7), which is 

 very commonly found attached to sea- weeds, resem- 

 bles Tabelldria in the frustules having the vittae 

 (PI. VI. fig. 8/). The frustules form short filaments, 

 attached by a little gelatinous cushion. The valves 

 have transverse striae, interrupted in the middle 

 (fig. 8 *). 



Gyrosig'ma (Pleurosig ma) anguldtum (PL VI. fig. 17) 

 resembles G. attenudtum in the sigmoid form ; but the 

 markings consist of lines crossing each other obliquely; 

 and these are resolvable into rows of dots (fig. 17 a) 

 under suitable illumination. 



In Coscinodis'cus radidtus (PI. VI. fig. 3) the frus- 

 tules are disk-shaped, the valves being elegantly 

 sculptured with easily recognized cell-like markings 

 or dots, so as to resemble a piece of vegetable cellular 

 tissue. But in some other species the dots are very 

 minute, and difficult to be shown satisfactorily. 

 These markings consist of depressions or pits in the 

 surface of the valves. That this is the case may 

 easily be seen by examining a fragment of the valve, 

 when the shadings of the broken ends of the netted 



