DIATOMACE.E 



6i 5 



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. Their form is usually 

 that of ellipsoidal discs, with surfaces more or less exactly parallel, 

 plane, or slightly curved ; and they are very commonly found ad- 

 herent to each other. The frustules in this genus are frequently in- 

 vested by a membranous envelope which forms a border to them ; but 

 this seems to belong to the immature state, subsequently disappear- 

 ing more or less completely. 



FIG. 4BO.Bhiso- 

 solenia imbri- 

 cuta. 



FIG. l&L.Achnanthes 

 longipes : a, b, c, d, e, 

 frustules in different 

 stages of binary divi- 



FIG. 462. Gomphonema gemi- 

 natum : its frustules connected by 

 a dichotomous stipe. 



Another family in which there is a dissimilarity in the two 

 lateral surfaces is that of the Achnanthece, the frustules of which 

 are remarkable for the bend they show in the direction of their 

 length, often more conspicuously than in the example here repre- 

 sented. This family contains free, adherent, and stipitate forms, 

 one of the most common of the latter being Achnanthes longipes 

 (fig. 461), which is often found growing on marine algae. The 

 difference between the markings of the upper and lower valves is 

 here distinctly seen ; for, while both are traversed by striae, which 

 are resolvable under a sufficient power into rows of dots, as well as 



