CRYPTOGAMOUS PLANTS. 241 



truding laterally. It opens at maturity by a true circu- 

 lar dehiscence, which takes place near its apex; the 

 upper part, which resembles the cover of a vessel, has 

 received the name of Operculum ; it is slightly depressed 

 at the margins, and conically elevated at the centre. 



After the fall of the operculum, we see that the inner 

 border of the theca is furnished with one or two mem- 

 branes terminating in regular teeth ; these membranes 

 bear the name of Peristome (peristoma), because they 

 surround the opening of the theca. The peristome, 

 when there is but one, or the outer one when there are 

 two, is very remarkable for its diversities of form and its 

 irregularity ; in a small number of cases it bears no 

 teeth, as in Gymnostomon ; most frequently it is bor- 

 dered with teeth or cilia, which are always equal and 

 of the number four, or one of its multiples, four in 

 Tetraphis, eight in Splachnum, sixteen in Grimmia, 

 thirty-two, forty-eight, or sixty-four, in different species 

 of Polytriclium. In several cases each tooth is half- 

 divided by a fissure, as in Dicranum, and in the same 

 case where it is not found, we perceive traces of it under 

 the form of longitudinal lines. We might believe that 

 the number of teeth is as great in the normal state, and 

 that they are united two and two, three and three, four 

 and four, &c. The inner peristome only exists in a part 

 of the Mosses ; it is more membranous ; its margin is 

 divided into 8, 16, or 32 teeth, which are often more 

 \ unequal and irregular than those of the outer peristome. 



In some genera, as Polytriclium, the tops of the teeth 

 of the peristome are all united together by a transverse 

 membrane stretched over the entrance of the theca ; this 

 membrane bears the name of Epiphragm ; when it 

 exists, the seeds can only come out between the teeth. 

 In almost all the other genera the teeth are free, and are 

 endowed with a very decided hygroscopical motion ; they 



VOL. II. R 



