106 Evans AND Hooker: PERISTOME IN CERATODON PURPUREUS 
subcylindrical ridges already noted in the mature teeth (FIG. 17). 
The differences between the inner and outer portions become less 
and less marked toward the apices of the branches (FIG. 18). 
Upon the transverse walls in the outer peristomial layer thickenings 
are also deposited. These are continuous with the thickenings on 
the inner wall and give rise to the transverse ridges of the mature 
teeth (FIG. 19). In a young stage, seen in tangential section 
(FIG. 20), the transverse thickenings apparently broaden out the 
outer longitudinal ridges, and the same appearance is even more 
‘ex 
9 
A 
Ha 
ae 
Fic. 17-19.- Cross and radial a ase in the region of the teeth, showing the 
peristomial thickenings in their final stage of development, 300; 20, tangential 
view of young peristome, seen from the outside, X300; 21, basal portion of a fully 
developed tooth, seen from the outside, X300. 
striking in an older tooth (FIG. 21). Toward the base of the 
branches the transverse ridges grow wider and wider until finally 
some of them span the distance between the branches and coalesce, 
thus forming a continuous ridge across the entire tooth. Below 
this region the ridges are all continuous. If Philibert’s ideas 
regarding the homologies of the peristomial layers are accepted, 
it becomes evident that the transverse ridges just described corre- 
spond with those present on the inner surface of the outer peristome 
in the Diplolepideae. In Ceratodon purpureus, however, the 
