166 Mr. G. H. K. Thwaites on the Diatomacese ; 



chromes ; but it is, nevertheless, perfectly certain that somethinp: 

 analogous to it must take place ; for, excepting the mixture oi 

 the endochronies of two cells, the phajnomena here exhibited arc 

 of precisely similar character to what has been noticed in the 

 other Diatomacece. In the Meloseirea*, instead of a conjugation 

 occurring between two frustules, a change is observed to take 

 place in the endochrome of a single frustule — that is, a disturb- 

 ance of its previous arrangement, a moving towards the centre of 

 the frustule, and a rapid increase in its quantity : subsequently 

 to this it becomes a sporangium, and out of this are developed 

 sporangial frustules as in the other Diatomacea. A careful consi- 

 deration of these phenomena, coupled with the fact of conjuga- 

 tion of endochromes being necessary in other species of the same 

 natural family, leads to the opinion that there is great probability 

 of a process taking place in the one cell of the Meloseirece pre- 

 cisely similar in physiological character to the conjugation or 

 mixture of endochromes in other species. In some species of 

 Zygnema, to which genus reference has been before made, a con- 

 jugation takes place between contiguous cells in the same filament , 

 and the contents of such pair of conjugated cells necessarily oc- 

 cupied one cell previously to its fissiparous division : it is there- 

 fore not difficult to believe, taking into view the secondary cha- 

 racter of cell-membrane, that the two kinds of endochrome may 

 be developed at the opposite ends of one frustule as easily as in 

 two contiguous frustules, and that at a certain period a mixture 

 of these may take place, giving rise to the same phsenomena 

 which succeed conjugation in other Diatomacece. The unity of 

 plan which runs through the whole of nature forbids our enter- 

 taining the idea of a physiological, though there may be a struc- 

 tural, difference in the phsenomena of reproduction in such closely 

 allied productions as the several species of Diatomacece. 



It is unnecessary to enlarge upon the importance of the doc- 

 trines now enunciated, if they are, as I believe, correct ; but the 

 remainder of the paper will be more exclusively occupied with 

 observations on the genera and species to be described. 



The general mode of formation of the sporangia in the Melo- 

 seirece having already been adverted to, it will now be necessary 

 only to indicate the peculiarities exhibited in those species of 

 this family, which have been met with in the state of fructifica- 



* It would seem that in the BiddulphiecB there is the same absence of an 

 evident conjugation; for s'pecimens of Odontella poli/morpha, Kiitz. {Bid- 

 dulphial IcEvis, Ehr. and Bailey), kindly communicated to me by Professor 

 Harvey of Dublin, who received them from Professor Bailey, exhibit spo- 

 rangia, each evidently originating from the endochrome of a single cell, and 

 in the early stage of growth appearing as a dilatation of one cud of such 

 cell. 



