304 



DIATOMACEjE : — family cocconeide.e. 



tinguished as the inferior. This family consists but of a single 

 Genus Cocconeis, which includes, however, a great number of 

 species, some or other of them occurring in every part of the globe. 



Fig. 138. 



Fig. 137. 



Rhizosolenia 

 iiuUricata. 



Achnanthes longipex : a, b, c, 

 d, e, successive f rustules in dif- 

 ferent stages of self -division. 



Their form is usually that of ellipsoidal disks, with surfaces more 

 or less exactly parallel, plane, or slightly curved, and they are 

 very commonly found adherent to each other. The frustules in 

 this genus are frequently found invested by a membranous enve- 

 lope which forms a border to them ; but this seems to belong to 

 the immature state, subsequently disappearing more or less com- 



