Algals.] 



DIATOMACEiE. 



13 



doubtful creatui'es, observes, that one division of them, the CjTnbelleee, rapidly become 

 putrid, have a sihceous covering, and consequently their form is not altered in drying, and 

 they are not destroyed by fire. When in perfection they are generally brownish, and not 

 imfrequently become greenish when di*y ; they are usually of either a quadrilateral or 

 prismatic form, and often marked with streaks and dots. The Desmidiejc, on the 

 contrary, putrify very slowly, have not a siliceous coat, and therefore alter their shape 

 in drjiug. When m perfection they are generally of an herbaceous green colour, and 

 most frequently have the fragments divided into two portions resembhng each other in 

 form, but sometimes differing much as to size. " This diWsion is marked in Desmidium 

 mucosum merely by a shallow groove passmg roimd the joint, and in Desm. Swartzii 

 by notches in the angles, by wliich it is rendered still more apparent ; wliilst in 

 Euastrum the two portions are connected only by a central chord." (Ann. N. ^.11. 

 448.) In another place (lb. 13. 377) this accm-ate observer recognizes the universal 

 presence of starch among the Desmidiese, which, not being an animal product, seems to 

 settle the question of the vegetable natiu'e of at least that portion of Brittleworts. 



Natives of still waters, and oozy places in the northern parts of the woi'ld. 



The uses of these plants to man are .vmknowTj. 



GENERA. 



Suborder I. C vmbelle.e. 

 — Individuals quite 



. free, angular, sili- 

 ceous. 



P'rustulia, Ag. 



Cyciotella, Ktz. 



Hap/otella, Ktz. 



Cymhella, Ktz. 



Navicula, Bory. 

 Styllaria, Ag. 



Rhahdium, Wallr. 

 Meridion, Ag. 

 Licmophora, Ag. 



Ejcilariu, Grev. 



Psyjmatelln, Kutz. 

 Biddulphia, Gray. 

 Gomphonema, Ag. 



Cymbophora, Ktz. 



Paltoaophora, Ktz. 



Sphenophora, Ktz. 

 Eunotia, Ehr. 



Cocconema, Ehr. 

 Achnanthes, Bory. 

 Striatella, Ag. 

 Amphitetras, Ekr. 

 Isthmia, Ag. 

 Diatoma, i)C. 



Bacillaria, Ehr. 

 Tabellaria, Ralfs. 

 Tessella, Ehr. 

 FragUaria, Lyngb. 



Nematoplata, Bory. 



Temachium, Wallr. 

 Grammonema, Ag. 

 Tetracycliis, Ralfs. 

 Lysigonium, Lk. 



Melosira, Ag. 



Gaillonella, Bory. 



Vesiculifera, Hass. 

 OncobjTsa, Ag. 



Suborder II. Hydroli- 

 HEJE. — Indi\aduals en- 



closed in tubes, an- 

 gular. 



Encyonema, Ktz. 

 Dickiea, Berk. 

 Hydrolinum, Link. 



Schizonema, Ag. 



Monema, Grev. 



Girodella, Gaill. 



Spennogonia, Bonnem. 

 Homceocladia, Ag. 

 Gloiodictyon, Ag. 

 Hydrurus, Ag. 



Gluzella, Bory. 



CorradorKS , Gray. 

 Micromega, Ag. 



Cakothrix, Desv. 



Suborder III. Desmi- 

 DiE^.— Individuals cy- 

 lindrical. 



Pentasterias. 



Xanthidium. 

 Euastrum, Ehr. 



Cosmarium, Menegh. 



Eutomia , Harv. 



Heterocarpella, Turp. 

 Odontella, Ehr. 

 Closterium. 

 Titmemorus, Ralfs. 

 Micrasterias, Ag. 

 Staurastrum, Meyen. 

 Pediastrum, Meyen. 

 Spheerastrum, Meyen. 

 Helierella, Bory. 



Potareus, Rafin. 

 Crucigenia, Morren. 

 Selenaca, Nitsch. 

 Heliactis, Ktz. 

 Scenedesmus, Meyen. 

 Tessarthra, Turp. 

 Echinella, Ach. 

 Desmidium, Ag. 



Numbers. Gen. 45. Sp. 457. 



Position. 



Acrita. 



DiATOMACE^. — Confervacese-Palmellese. 



