220 DESCETPTioTf OF \^Po7'i/gasfrica. 



of organic matter, but, fi-om their numbers and the indestructibility 

 of their shells, they have led to many important discoveries. The 

 large masses of meteoric paper which fell in 1686 have recently 

 been shown to consist mainly of their coverings, while several hun- 

 di'ed square feet of a flannel-like substance, lately foimd near Sabor, 

 in Silesia, after an inundation, was composed of Flagilaria, Navicula, 

 Crypfomonas, and Chsterina, interwoven with Conferva livularis. 



As microscopic objects, the markings of the fossil species are 

 highly interesting ; and when mounted as opaque objects, a verifi- 

 cation of the structure of their shell is clearly demonstrated. 



Ehrenberg's primary division of this family is into single and 

 double-loricated animalcules ; the genera comprised in the first being 

 again separated into those which are attached, by a pedicle or stalk, 

 and those which are destitute of siich appendage, or free. 



