8 - INTRODUCTION. 
gether, and towards the apex of the tooth they become confluent 
with each other; the line of their cohesion in the lower portion 
of the tooth is called te linea media or divisuralis. The in- 
ternal band equals or sometimes exceeds in width the united 
external bands: internally it presents a very convex surface, with 
its transverse articulations or trabecule frequently very promi- 
nent; externally its surface is plane, and to this the external 
bands everywhere cohere. When the internal band exceeds in 
width the external, the teeth appear as if bordered with a hyaline 
membrane. Sometimes the two external bands are separated 
along the medial line; and the teeth then appear to have a hya- 
line mark or fissure. At their base the teeth are combined with, 
and appear to originate from, the internal wall of the capsule. 
Within the external peristome just described, and adhering to its 
base, is a thin transparent membrane, composed at its base of 
eighty quadrangular cells; this membrane, the peristomium in- 
ternum, is regularly divided into sixteen wider segments, each 
two cells wide at its base, and carinate outwardly along the 
line by which they cohere; or more frequently they are per- 
forated along this keel, and cohere only by the laterally promi- 
nent articulations, or not at all; these segments are the processes 
of the inner peristome, and are arranged alternately with the 
teeth of the external peristome, so that the keel of one of the 
processes comes between each of the teeth. Interposed between 
each of the processes are three very narrow cilia, each of which is 
one cell wide. The processes and cilia are usually of equal 
length with the teeth, and are pale and membranous ; very rarely 
the processes are coloured. Such being the structure of the 
most complete or normal peristome, it will be necessary to con- 
sider its modifications, and those states in which some of the 
constituent parts are reduced or become entirely obsolete. In 
the greater number of species with complete peristomes, the 
apices of the processes are entire, like those of the teeth, from 
the confluence of the upper cells; but in some cases the pro- 
cesses are divided throughout their length into two divergent 
segments. This occurs in Acidodontium and in Bartramia. The 
cilia are sometimes confluent into one, and are often altogether 
obsolete ; the entire internal peristome may be only a membrane, 
irregularly divided by the rudiments of abortive processes, or 
may be wanting altogether: the external peristome, too, may be 
observed in many incomplete states, the internal band being so 
