272 
opposite or alternate, but between these two conditions there is 
found all possible intermediate stages (strictly opposite, opposite, 
subopposite, subalternate, alternate, strictly alternate), and in many 
species a certain variation is found even within the same colony 
The internodes may be of very different length, bearing one to 
many pairs of hydrothecæ, and also in this respect many. species 
present great" variation within the colony. When the internodes 
are very short the hydrothecæ must of course be opposite, and the 
alternate hydrothecæ, therefore, must be found in longer internodes, 
but the latter are not rarely provided with opposite hydrothecæ 
(Thujaria lichenastrum Pall., Th. sinuosa Bale e.t.c,). In opposition 
to what is said in the diagnosis we very often in species of 
Nutting's Thujaria find internodes with a single pair of hydrothecæ, 
and the author f. inst. figures branches of Th. polycarpa, Th. 
argentea and Th. tenera, which bear a series of 2—4 such inter- 
nodes. In Sertularia grisea Kirch, which is provided with inter- 
nodes bearing 1—5 pairs of subopposite hydrothecæ, I have seen 
brånches with up to 12 such internodes. According to the diagnosis 
of Sertularia there may be found in the genus subopposite hydro- 
thecæ, but when the internodes bear more pairs of hydrothecæ 
the latter are said to be strictly opposite. I do not understand 
why they may not be subopposite, but in either case they seem 
to be so in a rudiment of Sert. Challengeri figured by the author. 
It is evident, that the different arrangement of the hydrothecæ 
cannot give us a distinct delimitation between the two genera, and 
if we use the different length of the internodes we meet with the 
same difficulty when we try to draw a boundary line; but even if 
it were possible by means of the above characters to divide the 
species into two sharply separated groups, the latter would still 
be artificial, if we paid no attention to the structure of the hydro- 
thece and both groups contained species belonging to different 
natural genera. 
Sertularia (L.) Nutting. 
We may first regard the inconstancy in the structure of 
