EvANS AND HOOKER: PERISTOME IN CERATODON PURPUREUS 109 
Ridges of thickening, representing the future teeth, are laid 
down upon the periclinal walls between the two peristomial layers. 
The cells of the peristomial layers form eight groups, each 
composed of two rows of cells of the outer layer and three rows 
of the inner layer. Each group gives rise to two teeth. 
In the upper part of each group eight deposits of thickening 
are laid down in four strands, representing the four branches of 
the two teeth; in the lower part only two strands are formed, 
representing the basal undivided portions of the teeth. 
In the outer peristomial layer thickenings are deposited also 
upon the transverse walls, representing the transverse ridges of 
the teeth. 
In the undivided basal portion of each tooth a fine median 
longitudinal line on the inner surface represents the vestiges of the 
radial walls between two rows of peristomial cells. 
In the basilar membrane the thickening of the walls in the 
outer peristomial layer is uniform except in case of the outer walls. 
YALE UNIVERSITY. 
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