166 Mr. G. H. K. Thwaites on the Diatomacese ; 
chromes ; but it is, nevertheless, perfectly eertain that something 
analogous to it must take place ; for, excepting the mixture of 
the endochromes of two cells, the phsenomena here exhibited are 
of precisely similar character to what has been noticed in the 
other Diatomacecp. In Meloseirece^j 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 Diatomacece. A careful eonsi- 
deration of these phsenomena, eoupled with the faet 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 eell 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 eon- 
jugation takes place between contiguous cells in the same filament, 
and the eontents of such pair of conjugated cells neeessarily oc- 
cupied one cell previously to its fissiparous division : it is there- 
fore not difficult to believe, taking into view the seeondary eha- 
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 plijenomena 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, eorrect ; 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 neeessary 
only to indicate the peculiarities exhibited in those speeies of 
this family, which have been met with in the state of fruetifica- 
* It would seem that in the DiddulphiecE there is the same absence of an 
evident conjugation ; for specimens of Odontella jjolymorpha, Kiitz. (^Bid- 
dulphia'l IcBvis, 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 end of such 
cell. 
