OF THE FAMILY BACILLARIA. 



115 



bb. Without articulations. 



aa. Straight ribands, Fragillaria.* 



hb. Spiral ribands, Meridion.* 



B. Fixed, .... Eclunellea, 



a. Wider than long, Isthmia. 



b. Longer than wide. 



a. Without a pedicel. 



1. Prismatic, Synedra.* 



2. Wedgeform, Podosphenia.* 



b. Supported by a pedicel. 



1. Wedgeform. 



aa. Dichotomous, Gomphonema.* 



bb. Verticillate, • . Echniella.* 



2. Fixed perpendicularly, Cocconema.* 



3. Fixed obliquely. 



aa. Opening in the middle, Achnanthes.* 



bb. Without opening in the middle, . . . Striatella.* 



B. Carapace Double, .... Lacernata. 



A. Surrounded by an amorphous gelatinous mass. 



a. Scattered, Frustulia. 



b. Joined in rings, Syncyclia. 



B. Surrounded by membranous tubes. 



a. Tubes separate. 



a. Frustules straight, Naunema.* 



b. " curved, Gloenema. 



b. Tubes joined. 



a. Fasciculate, Schizonema.* 



b. Branching, Micromega. 



The genera marked thus (*) have been detected in the United States. 



Ehrenberg remarks of this family, that " the organization is difficult 

 to recognize, in consequence of the hardness and refraction of the 

 carapace. None have yet been found with calcareous coverings, 

 but they are either hard and siliceous, (sometimes containing a little 

 iron,) or membranaceous w^ithout silica. The differences observed 

 in the forms of the carapace have been made use of in classifying 

 them, (see preceding table.) In several genera, are found internal 

 hyaline vesicles of variable form ; these are colorless, and resem- 

 ble the stomachs of the Polygastrica, and in recent experiments 

 they have been colored by means of indigo. The female organs 

 are colored or colorless granules, forming two or four groups, which 

 are placed near the middle of the body, as in Navicula, Cocconema, 

 Nauneraa, &c. These eggs are frequently divided into several glob- 

 ular groups, which finally unite in a cruciform manner, (Achnanthes) 

 orwliich become confounded together, before being emitted, (Galli- 



