ON THE DEVELOPMENT AND STRUCTURE OF MOSSES. 25 



the vertical row of cells (Plate 1, fig. 8 h), which is continued 

 within the lid from the corresponding row (k) in the outer 

 wall of the space intermediate between the wall of the sporan- 

 gium and the spore-sac, and partly of the contiguous walls of 

 the cells (g} which spring immediately from the apex of the 

 intermediate space (i). The inner peristome is formed from 

 the thickening of the opposite sides, more towards the interior 

 of these last cells (g) and the contiguous cell- walls (/), which 

 are continued from the cells of the inner wall of the inter- 

 mediate space (/). In other words, if we suppose the outer 

 and inner walls of the intermediate space, consisting each of 

 a single layer of cells, to be continued above, but connected 

 beyond its apex by a single layer of cells (g), the outer wall 

 of the cells uniting the two walls continued upwards of the 

 intermediate space, together with the inner wall of the conti- 

 guous cells (h), will, when thickened, give rise to the outer 

 peristome; while the inner wall of the uniting cells (ff) I and 

 the outer wall of the cells continued from the inner wall of 

 the intermediate space will yield the inner peristome. 



In some cases, where the whole wall is not thickened, but 

 two or three thickenings take place in the same cell-wall, with 

 intermediate free spaces, the number of teeth is deranged as in 

 Tortula, but if two are formed, the fringe will still be symme- 

 trical (Crypt. Bot. fig. 98 a). Sometimes the inner teeth arise 

 at the angles of four contiguous cells, and if so, they will 

 generally alternate with the teeth of the outer peristome. 



Sometimes, as in Tetraphis (Plate 19, fig. 8), the tissue 

 within the lid splits up into four solid masses, which form the 

 teeth, while in Polytrichum, the teeth, which are continuous 

 with the membrane which closes up the ripe spore- sac, con- 

 sist of several layers, which in the Antarctic Dawsonia, where 

 there is no diaphragm, become distinct from each other and 



