

for the Microscope. 27 



at the centre. A number of vesicles are scattered over 

 the frond, ana not arranged in a single line as in the next. 



(PL III., fig. 22.) 



194. BEADED CLOSTERIUM (Closterium monilifcruui). 

 Smaller than the last, similarly swollen at the centre, 

 but it has a single row of vesicles down the middle. (PL 

 III, fig. 23.) 



195. LINED CLOSTERIUM (Closterium striolatuni). 

 When the green con tents are discharged, the membrane is 

 distinctly striated. It is a slender species, crescent-shaped, 

 with a single row of vesicles, and the ends are very blunt. 

 (PI. III., fig. 24.) 



196. BEAKED CLOSTERIUM (Closterium restrain HI). 

 This differs from all the other species we have enumerated 

 in tapering at each end into a kind of beak. The green 

 endochrome is confined to the central swollen portion, 

 which is about the length of one of the uncoloured ends. 



197. SPIRAL DESMID (Spirototnia condensatd). This 

 may be known from all the others which are here enume- 

 rated by the green contents forming a broad spiral band. 

 The sides are straight, and the ends rounded. It is a very 

 attractive species, and is common. (PL III., fig. 25.) 



FRESH-WATER DIATOMS. 



Of all microscopic objects Diatoms have engrossed the 

 most attention, and maintained the greatest popularity. 

 These organisms are plants, low in the scale it is true, but 

 still plants, belonging to the water-weeds or alga. One 

 great peculiarity in these minute plants is the possession of 

 a flinty or siliceous skeleton or framework, which is in 

 reality the portion sought after by microscopists. This 

 framework in most instances consists of two corresponding 

 valves, or more or less flattened plates, which are applied 

 together and form zfrustule, of which the front view and 

 the sieU view differ. These terms are employed in the 



