98 Evans AND HooKER: PERISTOME IN CERATODON PURPUREUS 
each of which in cross section shows two cells of the outer layer 
lying opposite one cell of the inner layer. One of the sixteen 
teeth of the outer peristome arises in each group and is formed by 
the deposition of: longitudinal bands of thickening upon the 
periclinal walls between the peristomial layers. These bands 
taper to fine points from a broader base. The band deposited 
by the two outer rows is of fairly uniform thickness throughout 
and extends as a continuous layer across the thin radial and 
transverse walls separating the cells. The band deposited by the 
single inner row corresponds in position and in width with the one 
just described and is of about the same thickness. Thickening is 
deposited also upon the transverse walls separating the cells of 
this row. Upon the disappearance of the portions of the walls 
which have remained thin the teeth show their mature condition. 
Each tooth bears upon its inner surface a series of transverse 
ridges corresponding to the thickened transverse walls, while the 
smooth outer surface shows the vestiges of the transverse and 
radial walls, the latter appearing as a zigzag median line running 
the entire length of the tooth. The delicate inner peristome is 
formed by deposits of thickening upon the inner walls of the inner 
peristomial layer. Toward the base the thickening is continuous, 
but in the upper part it is in the form of isolated longitudinal 
bands, definite in position and in number. At maturity the 
continuous portion forms the basilar membrane of the peristome, 
while the isolated bands form the segments and the cilia. 
In the Haplolepideae the peristome is likewise derived from 
two concentric layers of cells, but in this case each tooth is formed 
by one row of outer cells and two rows of inner cells and therefore 
shows the zigzag longitudinal line on the inner surface instead of 
on the outer. In the opinion of Philibert (’88, p. 68), the single 
peristome of this group is homologous with the inner peristome of 
the* Diplolepideae. This being the case it is evident that the 
large-celled inner peristomial layer in the Diplolepideae would be 
homologous with the large-celled outer layer in the Haplolepideae. 
The subject of the present study, Ceratodon purpureus, is a 
characteristic member of the Haplolepideae, and many of the 
features which the peristome shows, in development as well as in 
structure, are undoubtedly common to other species of the group. 
