MICROSCOPIC FORMS OF VEGETABLE LIFE. 



285 



principal Sections; one (B) containing those forms in which the valves 

 possess a true central nodule and median longitudinal line (as Pleuro- 

 sigma, Fig. 165, and Gomphonema, Fig. 186, A); and the other (A) 

 including all those in which the valves are destitute of a central nodule 



!as Surirella, Fig. 175, A). Among the latter, however, we find some 

 b) in which there is an umbilicus or pseudo-nodule with radiating lines 

 or cellules, whilst there are others (a) which have no central marking 

 whatever. 



284. Commencing with the last-named division (A), the first Family 

 is that of Eunotiece, of which we have already seen a characteristic ex- 

 ample in Epithemia turgida (Fig. 168). The essential characters of this 

 family consist in the more or less lunate form of the frustules in the 

 lateral view (Fig. 168, B), and in the stria9 being continuous across the 

 valves without any interruption by a longitudinal line. In the genus 

 Eunotia the frustules are free; in EpitJiemia they are very commonly 

 adherent by the flat or concave surface of the connecting zone; and in 

 Himantidium they are usually united into ribbon-like filaments. In the 



TIG. ITU. 



FIG. 170. Meridian circular e. 



FIG. 171. Bacillaria paradoxa. 



Family Meridiece we find a similar union of the transversely-striated in- 

 dividual frustules; but these are narrower at one end than at the other, 

 so as to have a cuneate or wedge-like form; and are regularly disposed 

 with their corresponding extremities always pointing in the same direc- 

 tion, so that the filament is curved instead of straight, as in the beautiful 

 Meridian circulars (Fig. 170). Although this plant, when gathered and 

 placed under the microscope, presents the appearance of circles overlying 

 one another, it really grows in a helical (screw-like) form, making several 

 continuous turns. This Diatom abounds in many localities in this 

 country; but there is none in which it presents itself in such rich luxuri- 

 ance as in the mountain-brooks about West Point in the United States, 

 the bottoms of which, according to Prof. Bailey, "are literally covered 

 in the first warm days of spring with a ferruginous-colored mucous mat- 

 ter, about a quarter of an inch thick, which, on examination by the 

 microscope, proves to be filled with millions and millions of these ex- 

 quisitely-beautiful silicious bodies. Every submerged stone, twig, and 

 :Spear of grass is enveloped by them; and the waving plume-like appear- 



