ilfr. Brown on Lyelliu, Leptodomiifn, and Buxbaum'ui. 57 1 



Allhougli there is but little resemblance in the structure of the 

 peristomiuni among the difFerent genera of Po/?/<r/cAoiWe<?, they may 

 stHl bo said essentially to agree in the function of this part: for in 

 all of them the complete separation of the seeds is ensured by the 

 emallness of the apertures for their discharge. It may be re- 

 marked, that the necessity for this complete dispersion in Mosses 

 seems to be inversely as the size of the seeds. For in those ge- 

 nera of the order in which the capsule either bursts irregularly or 

 has a naked mouth, the seeds are in general larger than in those 

 with a single, and still more manifestly than in those with a double, 

 pcristomium. And in conformity with this also, in Polijtrichum 

 tindulatum and lavigatum the tympanum is sooner ruptured or 

 removed than in the other species of the genus. 



The result of this comparison of Poli/triclium with Lyellia and 

 Datpsonia, although it confirms the propriety of their approxima- 

 tion, does not afford any clearly distinguishing mark for the very 

 natural section of the order which these three genera form. In 

 the mean time, however, it may be circumscribed, though not with 

 absolute precision, by a combination of the more general charac- 

 ters which have been now enumerated. 



LEPTOSTOMUM. 



In defining this genus, which was first proposed in my former 

 paper on Mosses, I relied chiefly on the undivided annular pro- 

 jection of the inner membrane of the capsule. I was induced 

 to employ this modification of the peristomiuni as a character, 

 though certainly far from being obvious, in finding it to exist in 

 several mosses of the southern hemisphere, having a similar and 

 peculiar habit; and which, had it been neglected, I must have 

 referred to Gymnostomum, with the greater number of whose spe- 

 cies they have hardly any thing in common. 



yoL. xn. 4 J5 Mr. 



