EVANS AND HOOKER: PERISTOME IN CERATODON PURPUREUS 107 
ridges become adherent to the inner walls of the peristomial layer 
instead of to the outer walls, as is the case in the Diplolepideae. 
In the lower part of a tooth the longitudinal ridges on the outside 
also show an increase in thickness until they extend more than 
half way across the cavities of the cells in which they are deposited 
(FIG. 22, on left). In the basal undivided portion of a tooth these 
ridges finally coalesce and form a single broad ridge (F1G. 22, 
Fic. 22~26. Cross and radial sections in the region of the basal portion of 
the peristome, X300. 22 and 23, showing the portions of two teeth; 24 and 25, 
showing the same, together with the basilar membrane; 26, showing the basilar 
membrane only. 
on right, 23), beyond which the transverse ridges project for only 
a short distance (FIG. 24, 25). 
The thinner deposits of thickening formed by the inner per- 
istomial cells gradually increase in width toward the base until 
they finally meet and coalesce at the radial walls between the two 
rows of cells (FIG. 17, 22, 23). The vestiges of the radial walls © 
form a zigzag longitudinal line on the inner surface of the tooth, 
and not on the outside as in the Diplolepideae. This line is of 
course much shorter than in Funaria hygrometrica and Mnium 
hornum, in which the teeth are not divided into branches. No 
