88 



Bacillariese 



striations or areolations. A few diatoms are known in which the valves are 

 smooth, no trace of striations or other markings having been discovered. 

 Such diatoms, in which the wall is very thin and apparently structureless, 

 are exemplified by Navicula perlepida, N. glaberrima, and Tropidoneis 



ima. 



Detailed microscopical investigation has shown that the markings consist 

 of small cavities within the siliceous cell-wall which are in most cases 



,-S 



Fig. 61. Triceratium Favus Ehrenb. A, valve view; J3, transverse section across the rim of the 

 valve ; C, girdle view ; D, surface view of portion of valve to show details of structure. 

 b, limitations of broken part of valve to show the hexagonal chambers (ch) o, external 

 openings of the large chambers ; p, pore ; po, poroids or dots ; s, spines at the corners of the 

 honeycomb-like chambers ; w, external wing at the rim of the valve. (B after 0. Miiller ; 

 C after W. Smith ; D after Pfitzer.) 



arranged in regular rows, thus giving the appearance of striations. In some 

 cases the marks are due to ridges, and both the cavities and ridges may 

 be either upon the inner or the outer side of the cell-wall. When the 

 cavities giving rise to the appearance of striations are very small they 

 are known as punctw. The striae vary in strength from conspicuous ribs 

 or costte to lines so fine as readily to escape detection, and species with 



