448 Mr. J. Ralfs on the Diatomaceae. 



division of the Algse (except in the Schizonemata, to which genus 

 Mrs. Griffiths and Mr. Harvey have more particularly directed 

 their attention,) we had latterly made much less progress than in 

 the others, and indeed that we were now far behind the conti- 

 nental observers. 



Having during the last two years paid much attention to the 

 subject, and examined specimens from as many habitats as pos- 

 sible, I conceive that I have acquired some useful information 

 respecting this interesting tribe, and intend from time to time to 

 draw up my observations on such genera and species as have fallen 

 under my notice. But I must preface what I have to say re- 

 specting them, by mentioning the names of those friends who 

 have in various ways assisted me in this pursuit. 



I wish to take this and every opportunity of acknowledging the 

 invaluable assistance I have at all times received from Mr. Borrer, 

 and without which I should not have been able to have studied 

 these microscopic plants with any success. 



To the Rev. M. J. Berkeley I have been indebted for his obser- 

 vations on many species, and also for portions of specimens taken 

 from the collections of dried Algse published by Jurgens and Des- 

 mazieres. 



Not having access to Ehrenberg's work on the Infusoria, my 

 thanks are due to Mr. Dalrymple for numerous extracts and co- 

 pies of many of the figures, as well as his assistance in identifying 

 several of his species. 



The Diatomaceae (Bacillaria^hrenb.) form one of the four great 

 series into which the Algse are divided, and comprise plants be- 

 longing to two very distinct families, Desmidiece (Desmidiacea, 

 Ehrenb.) and Cymbellece (Naviculacea, Ehrenb.) *. 



The Desmidiece have a membranous covering destitute of silica, 

 and their form is consequently altered in drying. When in per- 

 fection they are generally of an herbaceous green colour, and most 

 frequently have the frustules divided into two portions, resem- 

 bling each other in form but sometimes differing much in size. 

 This division is marked in Desmidium mucosum merely by a shal- 

 low groove passing round the joint, and in Desmidium Swartzii 

 by notches in the angles, by which it is rendered still more appa- 

 rent ; whilst in Euastrum (Eutomia, Harv.) the two portions are 

 connected only by a central chord. 



To this family belong the following genera, so far as regards 

 our British flora : Desmidium, Staurastrum, Pentasterias, Tessar- 

 thra } Xanthidium, Scenedesmus, Micrasterias and Euastrumf. 



* See Pritchard's History of the Infusoria, p. 173. 



f If Closterium be considered to belong to the vegetable kingdom it may 

 find a place in this family ; for my own part, I think the view taken by Meyen 

 is the correct one. For some interesting observations by him on this subject, 

 see Pritchard's ? History of the Infusoria/ p. 179. 



