296 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



186. In the group at which we now arrive, there is more or less 

 of permanent connection between the frustules themselves ; and 

 this may depend merely upon the cohesion of their surfaces, or may 



be occasioned by the persis- 



FIG. 91. tence of the connecting 



membrane of the valves after 

 the completion of the self- 

 division of the frustules. To 

 the former division belong 

 several genera, the form of 

 whose frustules is more or 

 less elongated, so that the 

 filament formed by their co- 

 hesion is a flattened band. 

 If the two extremities of the 

 frustule be of equal breadth, 

 as in Bacillaria,) the band 

 will be straight; but if one 

 be broader than the other, 

 so that the frustule in front 

 view has a cuneate or wedge- 

 like form, the filament will 

 be curved, as in the beauti- 

 ful Meridian circulars (Fig. 

 92, A). Although these, 

 when gathered and placed 

 under the microscope, pre- 

 sent the appearance of circles overlying one another, they really 

 grow in a helical (screw-like) form, making several continuous 



FIG. 92. 



A, Meridian circulare : B, Bacillaria paradoxa. 



turns. This Diatom abounds in many localities in this country; 

 but there is none in which it presents itself in such rich luxu- 

 riance, as in the mountain brooks about "West Point in the 

 United States, the bottoms of which, according to Prof. Bailey, 



