INTRODUCTION. Wl 
reduced in thickness as to be imperceptible, and the teeth ap- 
pearing to consist of the external bands only. Species occur in 
many natural genera which are either entirely destitute of pe- 
ristome, and termed gymnostomous, or which only produce that 
organ under certain favourable conditions. 
There are also a number of diminutive species which have the 
capsule constantly closed; in some of these the divisural line at 
which the operculum should separate is clearly marked, in others 
this line is entirely wanting. These species have usually been 
considered to form a section apart from the operculate Mosses, 
under the name of Cleistocarpi; but they have less of affinity 
with each other than they have with the species of several oper- 
culate and peristomate genera, with which they are connected by 
those species which have a defined although persistent opereu- 
lum ; and in the arrangement here used they have been placed 
in juxtaposition to those genera to which they are most closely 
related in all other particulars of habit and structure. 
One other form of peristome remains to be considered, in 
whieh the teeth, about thirty-two or sixty-four, but not exactly 
constant in number, are entirely composed of exceedingly narrow 
elongated agglutinated filaments. In these Mosses, the Nema- 
todontes, either the thin short teeth cohere by their apices with 
a tabular expansion of the top of the columella, and the mouth 
of the capsule therefore appears as if closed by a membrane, or 
the filaments are entirely free, and form a brush with which the 
mouth of the capsule is filled; or they may be united into a 
folded membrane. 
The whole of the Musci being therefore capable of division 
into a few primary sections by the structure of their fructifica- 
tion, such other characters as are afforded by habit or especial 
modifications in the evolution of the peristome become available 
for use in distinguishing the different tribes and genera, in the 
following manner :— 
CONSPECTUS TRIBUUM. 
Subordo I. SrEGocanPr. Theca supra medium linea transver- 
sali aperiens, parte superiore caduca rarius persistente. 
A. Homodictyi. Foliorum areolatio e cellulis homomorphis. 
1. Elasmodontes. Peristomii dentes 4, e cellulis pluribus conflatis 
compositi, articulationibus transversis 0. 
