1 9 8 DIA TOM A CE^E. PART n. 



The diatom or frustule is considered to present its 

 front view when the joint or suture of the valves is 

 turned to the eye, as in fig. 11 B, fe, whilst the side 

 view is seen when the centre of either valve is directly 

 beneath the eye, as in fig. 1 1 A, a. When the diatoms 

 are young the valves are in close contact, but as they 

 increase in size by a secretion round their edges, the 

 valves separate from one another, and the cell membrane 

 which is left exposed is immediately consolidated by 

 silex, and forms a kind of hoop between 

 the valves, as in fig. 12. This hoop in- 

 creases in breadth as the cell increases in 

 length. When the two valves are circular 

 discs, they are separated by a circular hoop, 

 round the edges of which water is admitted 

 Fig 12. Biddui- to nourish the plant ; but when the diatom 



plua pulchella. 



has an elongated form, the water enters 

 through depressed points in its extremities which are 

 free from silex. 



Numerous as these plants are, the valves of each genus 

 have their own peculiar ornaments, consisting of the 

 most beautiful and symmetrical designs, which are 

 impressed upon the young valves when they are in a 

 plastic state. The genus Navicula and others are 

 marked with the finest striae, some diagonally, others 

 transversely. Rows of round or oval spots disposed in 

 parallel lines are peculiar to some ; the valves of others 

 are covered with hexagonal forms of the most perfect 

 structure, as those of the Pleurosigma angulatum, fig. 

 13, where A is the magnified diatom, and B and c its 

 hexagonal areolations, seen under higher and higher 

 microscopic powers ; but the figures on the discoid 

 genera are the most beautiful of all. There is gene- 

 rally a small ornamented circular space in the centre of 

 the valves, from whence rays extend to the circumfer- 

 ence, dividing the surface of the valves into eight, ten, 



