XIV. ALG^]. 979 



dividing into polymorphic fragments [frustules]. Diatomece have a rigid envelope, 

 marked with extremely fine stride, fragile, siliceous, 2-valved, remaining unchanged 

 when dry, containing a brown or yellowish nitrogenized matter, sometimes with a 

 rather rapid creeping movement. Some authors refer Diatomece to the Animal 

 Kingdom. Certain species are parasitic ; others form flakes, or gelatinous masses 

 upon rocks ; others live in fresh and pure spring-water ; others again cover the soil 

 with a more or less thick brown sticky layer, often occupying a considerable space. 

 They abound in public fountains, the walls of which they stain brown. 



PKINCIPA'L GENERA. 



Micromega. Encyoneraa. Diatoma. Exilaria. Cyclotella. 



Schizonema. Gaillonella. Achnanthes. Stigmatella. Asteromphalus. 



Homseocladia. Fragillaria. Cymbophora. Surirella. Spatangidium. 



Berkeleya. Meridion. Gomphonema. Frustulia. 



Besides their multiplication by spores, resembling that of DMmidiece, Diatomece are multiplied by 

 deduplication. On the centre of each frustule in the solitary species, and of each segment or joint in the 

 species of which the frustules are aggregated, there is frequently visible in the young Diatom a line or 

 stria dividing it into two or several frustules, which become distinct individuals, similar to the parents. 

 In the Diatoms with united frustules the effect of this deduplication is to multiply the number of its 

 segments. 



Many Dtatomeee have been found in a fossil state. Ehrenberg discovered that certain ' rotten-stones,' 

 employed in the arts, were entirely composed of their siliceous shells. M. de Brebisson has shown that 

 calcining living species neither destroys nor distorts their envelope, and that by this process an artificial 

 rotten-stone can be made. The so-called ' mineral flour,' to which geologists have given the name of 

 Diatomepelite, has the same origin. Diatomccc abound in guano. 



ALG^E SPURI^E. 



We have classed as Doubtful Algce (Algce Spurice) certain imperfectly known 

 genera, and which are probably only degraded forms of the preceding families : such 

 are the families Rivulariece, Oscillariece, Nostochinece, Palmellece, and Volvocinece. 



They are gelatinous Algce of a whitish green, brackish, red or brown colour, 

 living on earth or damp stones, or in fresh either cold or thermal water, rarely in 

 the sea ; composed either of globules, or of simple or branched filaments, continuous 

 or chambered, nearly always enveloped in mucilage. 



[Many of their genera are now so familiar to us through the researches of microscopists, 

 that a brief notice of them seems to be required, under the tribes to which they have been 

 referred. 



I. OSCILLATOEIE^;, including RIVULARIEJ:. Plants growing in fresh and brackish, pools, 

 hot springs, rivers, vegetable infusions, &c., formed of transversely striated or moniliform 

 filaments, sometimes spirally curled or sheathed in mucus, exhibiting a serpentine motion. 

 Reproduction by transverse division. 



PRINCIPAL GENERA. 



Oscillatoria. Spirulina [? Spirularia]. Microcoleus. Lyngbya. Scytonema. 



Ulothrix. Calothrix. Sclerothrix. Rivularia. Glseotrichia. 



Euactis. Leptothrix. Bacterium. "V ibrio. 



3 K 2 



