156 FORAMINIFERA, RADIOLARIA, DIATOMS LESS. 



parts or valves (B, c, c. w, c. w'), each provided with a rim or 

 girdle, and so disposed that in the entire cell the girdle of 

 one valve (c. w) fits over that of the other (c. w'} like the 



-mi 



u_U c.u*> 



r.w 





FIG. 35. A, semi-diagrammatic view of a diatom from its flat face, 

 showing cell-wall (c. iv] and protoplasm with nucleus (mi], two vacuoles 

 (vac), and two chromatophores (chr\ 



B, diagram of the shell of a diatom from the side, i.e., turned on its 

 long axis at right angles to A, showing the two valves (c. "w, c. iv f ) with 

 their overlapping girdles. 



C, the same in transverse section. 



D, surface view of the silicious shell of Navicula truncata. 



]:, surface view of the silicious shell of \tilacodiscus sollittianns. 

 (n, after Donkin ; !:, after Norman.) 



lid of a pill-box. The cell-wall is impregnated with silica, 

 so that diatoms can be boiled in strong acid or exposed to 

 the heat of a flame without losing their form : the protoplasm 



