1G8 
Mr. G. H. K. Thwaites on the Diatomacese ; 
fig. A 2 a filament consisting of sporangial frustules produced 
from one sporangium. Fig. C is a filament of M. Borreri, con- 
sisting partly of ordinary and partly of sporangial frustules. 
M. nummuloides of Kiitzing I cannot help believing to be the 
sporangial state of M. salina of the same author. 
In Aulacoseira crenulata the sporangium is spherical^ with its 
axis of elongation at right angles to that of the frustule from 
which it originated. Around the young sporangium a consider- 
able quantity of mucus is developed, by which the empty half- 
frustules are for some time held attached. Fig. B 2 represents 
filaments of Aulacoseira crenulata with sporangia ; and fig. B 3 
sporangial frustules of the same species. 
Orthoseira Dickieii, n. sp. FI. XII. fig. E 1-7 {—). Filamentis 
brevibus ; cellulis Isevissimis. 
The filaments of this beautiful species consist generally each of 
from two to four frustules, which are hyaline and perfectly smooth, 
and each with its central cavity filled with a dark red-brown en- 
dochrome. The sporangium of Orthoseira Dickieii is no less 
beautiful than interesting : it is fusiform in shape and marked 
with numerous annular constrictions, each with a corresponding 
internal septum or chamber, the origin of which can only be un- 
derstood by paying attention to the early development of the 
sporangium. In fig. E 3 is shown a filament of this species, the 
terminal cells of which have each commenced to develope a spo- 
rangium ; E 4 represents two such cells or young sporangia ; 
andE 5 a mature sporangium. It will be observed that the for- 
mation of the ring-like markings is progressive, and that they go 
on increasing in number until the sporangium is fully developed. 
At the commencement of the formation of the sporangium, the 
endochrome, at the same time that it withdraws from the end of 
the frustule, produces at its centre an additional ring of cell- 
membrane ; and this process continuing to take place at certain 
intervals — each new ring of cell-membrane exceeding in diameter 
those previously formed — produces at length the structure re- 
presented in E 5. Or it may be a more correct explanation of 
the process to say, that an entire new cell-membrane has been 
developed by the young sporangium at the time each new ring 
has been formed, and that thus have originated the several 
chambers into which the ends of the sporangium are divided. 
Fissiparous division of the s})orangium subsequently takes place, 
as shown in fig. E 6, and sporangial frustules are developed from 
each half, E 7. 
Meloseira americana, Kiitz. Bacillarien, 55. tab. 30. fig. 69, is 
evidently congeneric with this species ; differing from it princi- 
pally in the ends of its frustules being striated. 
