ARCHER, ON DESM1DIACE.E. 217 



mutual affinities of organic objects; and while at the same 

 time I will not deny, in regard to certain organisms which 

 seem to be incongruously united with certain groups or 

 families, that it sometimes happens, while our present state 

 of knowledge as to their nature and history is deficient, that 

 it is more advisable to allow the puzzling forms to remain 

 combined with such groups as may appear temporarily the 

 most convenient; nevertheless, if any organism be found 

 really not to agree with the characters which are common to 

 and appear to pervade an apparently perfectly natural assem- 

 blage, it would seem to me to be repugnant to a proper 

 classification, if it could be avoided, that it should be there- 

 Avith associated. 



I shall, then, venture to delay, before entering on the sub- 

 ject proper of this communication, by drawing attention to the 

 diagnosis of this family, as given in Ralfs' monograph, " The 

 British Besrnidise :" — "Fresh-water figured mucous and 

 microscopic algse of a green colour. Transverse division 

 mostly complete, but in some genera incomplete. Cells or 

 joints of two symmetrical valves, the junction always marked 

 by the division of the endochrome, often also by a constric- 

 tion. Sporangia formed by the coupling of the cells and 

 union of their contents." Although I have no new obser- 

 vation in regard to the history of Pediastrum to add, I 

 shall just briefly compare that genus with the foregoing 

 definition. 



That Pediastrum agrees with the first clause of Ralfs' 

 diagnosis is indeed apparent. 



In regard to the second clause, so far as I can make out, I 

 believe the complete fission into two distinct cells of any of 

 the component cells has not been observed ; that is to say, I 

 believe the number of component cells in any particular 

 frond is not increased after their first formation; in other 

 words, there does not appear to be any extension of the cell- 

 wall of any cell accompanied by a transverse fission. Mr. 

 Ralfs mentions that he did not see cell -division. I have 

 certainly myself, so far as my own limited experience in this 

 genus goes, never noticed anything to indicate the mode of 

 division characteristic of the Desinidiaceee. By this, of 

 course, is not meant to be denied the subdivision of the 

 endochrome within the parent cell, — the necessary prelude 

 to its conversion into zoospores. The number of constituent 

 cells in a frond, of often indeed even the same species, seems, 

 therefore, to depend on the number of times the original 

 endochrome of the parent cell had become segmented, and 

 the consequent number of zoospores. Occasionally a frond 



