140 ULOTHRICACEAE AND CHAETOPHORACEAE 
viction that it is the only course based on rational principles and 
offering a fair hope of attaining the goal of stability. How little 
disturbance this method makes in these groups is seen in the fact 
that it has appeared necessary only in two cases to displace the 
names current among modern algologists, namely, Conferva and 
Stigeoclonium. 
Very few varieties have been listed, because experience has 
proved that many of those in current use are either mere growth 
forms of the species with which they are associated, or are wrongly 
associated with the species. Mr.Wolle was in the habit of listing all 
the varieties given by Rabenhorst and Kirchner, without reference 
to their actual occurrence in this country. These have been repeated 
by De Toni as American forms, and thus confusion has arisen. 
A sharp distinction between variety and forma, in the technical 
sense, has been here understood: by the term variety, better 
called subspecies, is indicated a form which is well-marked in re- 
lation to the species and fairly constant in character ; by the term 
forma we have designated a form which is sufficiently well marked 
to demand recognition, but which is regarded as probably only a 
growth state of the species. 
The limits of the two families treated are drawn practically on 
the lines laid down by Wille in Engler & Prantl, Die natürlichen 
Pflanzenfamilien. The chief departures from that work are the 
relegation of Trentepohlia and Acroblaste to a separate family, be- 
cause of their specialized sporangial cells, and the removal of 
Microthamnion from this group to what is clearly its proper place, 
among the Chaetophoreae. As a matter of convenience we have 
also arranged the microscopic genera of the Chaetophoraceae in a 
separate tribe, the Herposteireae, taking the name from the most 
highly developed of the group, a name which is at the same time 
most suggestive of the creeping habit of all the forms. 
It is very probable that the position taken by Borzi (’89), 
Bohlin (97), and Wille (or), in removing Conferva (= Tribo- 
nema) from the Ulothricaceae to a separate family and order in 
close affinity with the Ophiocytiaceae, is well grounded; for 
the present convenience, h wever, of American students who have 
not distinguished this genus from Microspora, it has been tempo- 
rarily retained in its old position. 
