108 Evans AND Hooker: PERISTOME IN CERATODON PURPUREUS 
transverse ridges are developed in the inner peristomial layer, 
but the vestiges of the walls remain visible as fine lines. 
When the strands of thickening have reached their full size, 
the spicules of cell wall substance are deposited in immense number 
over the entire surface (FIG. 17-19, 21-25). Those formed by 
the outer cells are considerably longer than those formed by the 
inner cells, but all are exceedingly minute. 
The cells which form the basilar membrane of the peristome 
acquire thick walls also, but the deposits of thickening differ from 
those which form the teeth. This is especially true of the outer 
peristomial layer where the thickening extends almost evenly over 
the inner walls, the radial walls, and the transverse walls but leaves 
large pits on the outer walls (Fic. 25, 26). In the inner per- 
istomial layer the thickening is less pronounced and forms a con- 
tinuous layer over the outer wall. Throughout this portion of the 
peristome the surface of the thickening remains smooth, no 
spicules being developed. The basilar membrane lies just within 
the large cells of the annulus (FIG. 25). 
Soon after the spicules have been deposited upon the teeth 
the peristomial cells dry up, the thin parts of the walls shrivel 
away and disappear, and the teeth become free. In the basilar 
membrane the pits in the outer wall now appear as perforations. 
At the same time the operculum becomes separated, and the 
endothecial tissues disintegrate. The finer structure of the 
mature peristome and the hygroscopic movements which it 
executes are fully described by Steinbrinck (’97). 
SUMMARY 
The original amphithecium, showing eight cells in cross 
section, divides by periclinal walls into an inner and an outer 
layer. 
The inner peristomial layer develops from the inner amphi- 
thecial layer, undergoing division by anticlinal walls until it is 
composed of twenty-four longitudinal rows of cells. 
The outer peristomial layer develops from sixteen longitudinal 
rows of cells cut off by periclinal walls from the outer amphithecial 
layer, after its eight rows have been divided by anticlinal walls; 
the outer peristomial layer undergoes no further divisions by 
anticlinal walls. 
