PROTOPHYTES. 



95 



The Hdiopelta, sun-shield, (from helios, the sun, and 

 pelta, a shield, (Fig. 33,) is a most beautiful disk, whose 

 markings form a regular star, the number of whose rays 

 determine the species. 



The Diatoma vulgare, (A., Fig. 34,) is a quite common 



FIG. 34. A. Diatoma vulgare : a. Side view of frustule ; b. Frustule undergoing 

 self-division. B. Grammatophora serpentina. a. Front and side views of single frus- 

 tule. b. b. Front and end views of divided frustule. c, A frustule about to undergo self- 

 division. </. A frustule completely divided. C. Isthmia nervosa. 



form. The frustules often hang together, forming zig- 

 zag chains by rapid self-division. 



Some species of Grammataphora have delicate striae 

 on the borders of each valve, which are used as tests of 

 microscopic excellence. 



The Isthmia nervosa has a remarkable areolated 

 structure, which will repay careful examination. In its 

 growth two cells form within the valves, and, as they 



