34 
F. Cavers. 
III. Buxbaumiales. Peristome double, arising in several 
(three to six) concentric cell layers of the amphithecium. Outer 
peristome (exostome) consists of from one to four series of slender 
teeth, the inner peristome (endostome) of a thin tubular membrane 
resembling a truncated cone and showing sixteen or thirty-two 
longitudinal folds. Both peristomes consist of thickened cell-walls 
only, not of entire cells, as in Tetraphidales and Polytrichales. 
In this respect the Buxbaumiales approach the Eu-Bryales, though 
the greatest distinction between the two groups lies in the fact 
that in Eu-Bryales the number of layers constituting the peristome¬ 
forming zone is more definitely limited. 
IV. Eu-Bryales. Here the peristome-forming zone consists 
either of two or three layers of cells, the walls of which become 
thickened in such a way as to form typically flat triangular teeth, the 
unthickened walls becoming resorbed, so that the teeth consist of 
plates representing the thickened portions of the walls of either two 
or three concentric cell-layers. The horizontal walls in some cases 
do not undergo complete resorption, so that the teeth are then 
chambered and appear in longitudinal section to consist of a row of 
cells, though these cells are open at either side of the tooth. The 
number of teeth is typically either sixteen or thirty-two, more rarely 
eight or sixty-four. Where resorption is incomplete (e.g., Splachnum, 
Splachnobryum, Leptostomum, Leucodon ) the peristome is regarded 
as being in reality double, and it is an open question whether this 
“ chambered” type of peristome is primitive or reduced. The Eu- 
Bryales are sub-divided, on the peristome structure, into three 
series. 
(A). Haplolepidece. Here the peristome consists of a single 
series of teeth, formed of membranes belonging to two concentric 
cell-layers. The outer face of each tooth shows a single row of 
membranes, while the inner face shows two series of membranes 
owing to the presence of the remains of the radial walls of the inner 
peristome-forming layer of cells. The sixteen teeth are either 
simple or are divided more or less deeply into two (sometimes three) 
lobes or filaments. The Haplolepidece are divided into five cohorts, 
differing with regard to the entire or lobed character of the teeth, 
the presence or absence of a collar-like “ properistome ” outside of 
the peristome, the relative thickness of the two membrane layers 
composing each tooth, the presence or absence of striations and 
papillae on the outer aqd inner faces of the teeth, the presence or 
