Oct. 1, 1869.J 



HARDWICKE'S SCIENCE-GO SSIP. 



221 



bottom has been replaced by a second lid, roughly 

 illustrates the perfect diatom aceous frustule: the 

 lids represent the valves, and the body of the box 

 the connecting baud or cingulum. To get rid of the 

 eudochrome, aud to effect the separation of the 

 valves, it is necessary to boil the gathering in nitric 

 acid : this will also destroy any vegetable or animal 

 matter mixed with the gathering. After boiling in 

 acid, the diatoms must be allowed to subside, and the 

 acid poured off, and replaced with distilled water. 

 It is highly important that all trace of acid be 

 washed away, particularly when the forms are too 

 delicate for balsam-mounting. 



It sometimes happens that in spite of careful 

 washing the diatoms have a tendency to form little 

 clusters, and cannot be evenly distributed on the 

 slide. This may be prevented by pouring off the 

 water, and replacing it with liquor ammonia. If 

 the diatoms are allowed to remain in this a short 

 time (shaking the bottle or test-tube occasionally), a 

 quantity of nocculent matter will be held in sus- 

 pension, and may be poured off with the liquor 

 ammonia, allowing a longer or shorter time, accord- 

 ing to size, for the subsidence of the diatoms. 



Our tyro, having cleaned and mounted his gather- 

 ing, will probably be desirous of knowing the names 

 of the forms occurring in it. I hope to enable him 

 to do so by the aid of the following figures and 

 descriptions of those species most commonly found 

 in the before-mentioned localities : — 



Fig. 173. 

 Navicula granulata, x 



400. 



Fig. 174. 

 Naricula humerosa, x 400. 



Navicula granulata (fig.173). — This form is readily 

 distinguished by its lines of conspicuous and brilliant 

 dots, reaching the median line and central nodule,* 

 straight or slightly rounded sides and pointed ends : 

 it always occurs in sand-gatherings. 



Navicula humerosa (fig. 174). — A species resem- 

 bling the preceding in outline, but the dots are appa- 

 rently replaced by slightly radiating lines : these lines 

 will, however, be found to consist of small dots placed 



* The valves in the genera Navicula, Pinnularia, Pleuro- 

 sigma, Cocconeis, and a few others, have a straight line or bar 

 crossing the larger diameter: this is called the median line. 

 In the centre of the line is a large dot called the central 

 nodule ; the ends of the median line terminate in two smaller 

 nodules. 



close together : common with the preceding ; it 

 sometimes occurs with the sides slightly concave. 



Navicula rectangulata (fig. 175).— A small, narrow 

 form, with straight sides and rounded ends : the 

 markings are radiant, distinct, and not resolvable 

 into dots ; gradually shortening as they approach 

 the central nodule, leaving a small oval blank space 

 in the centre of the valve. 



Fig. 175. 

 Navicula rectangulata, x 400. 



Fig. 176. 

 Navicula angulosa, X 400. 



Navicula angulosa (fig. 176). — This species is of an 

 oval form, the sides gradually tapering towards the 

 pointed ends ; the markings reach about halfway 

 towards the median line ; thus leaving a central 

 blank space, similiar in outline to the valve itself. 



Navicula Lyra (fig. 177). — This species is very 

 variable in outline ; sometimes it is a true oval, 

 at others the sides are straight, with the ends drawn 

 out, and blunt. Another variety has the sides 

 straight and tapering suddenly to the pointed ex- 

 tremities. This species has been named from the 

 lyre-shaped blank spaces on each half of the valve : 

 these are also subject to variation ; the ends are 

 sometimes curved inward instead of outward ; the 

 markings consist of closely-set dots, appearing like 

 lines under a low power. 



Fig. 177. Navicula Lyra, 

 X 400. 



Fig. 178. Navicula a>stiva, 

 x 400. 



Navicula rnstiva (fig. 178). — A very pretty little 

 species, of an oval form and pale straw-colour : the 

 markings are fine slightly radiant lines reaching the 

 median line. 



Navicula retusa (fig. 179, a, b).—K. [species in 



