342 ON LYELLIA, LEPTOSTOMUM, 



though it may be a natural genus, must be given up, until 

 other marks shall be found by which it may be distinguished. 

 I have not had specimens sufficiently perfect to enable me 

 to judge of the structure of all the species of Gymnostomum 

 mentioned by Mr. Hooker. In one of them, however, Gym- 

 nostomum microstomum, the peristomium is certainly very 

 different from that of Leptostomum. In this species I find, 

 on removing the operculum, that the mouth of the capsule 

 is not only completely covered by a horizontal membrane, 

 but that this covering is derived from the outer membrane 

 of the capsule, and consequently differs in origin as well as 

 in form from the peristomium which it has been said to 

 resemble. Its central portion, however, being extremely 

 thin is soon ruptured and deciduous, and in this state only 

 it has been seen by the authors of the Muscologia Britannica. 



Gymnostomum microstomum therefore may itself be con- 

 sidered as a distinct genus, to which the name of Hymenos- 

 tomum 1 may be given ; and it is worthy of remark, that in its 

 technical character it approaches most nearly to Lyellia, 

 though no two mosses differ more widely in almost every 

 other respect. 



573] From the account given of Gymnostomum Griffiitki- 

 anum it appears that this species also has in the early stage 

 a membrane completely covering the mouth of the capsule. 

 But this membrane probably differs in origin, at least, from 

 that of Hymenostomum, as it seems to do both in form and 

 dehiscence from the peristomium of Leptostomum. 



Of Gymnostomum fascicular e I have examined only im- 

 perfect specimens, I cannot therefore speak with confidence 

 of its structure. The annular process, however, mentioned 

 by Mr. Hooker is more likely to be the remains of a com- 

 plete horizontal covering, and probably originating from the 

 inner membrane, than to resemble the peristomium either of 



1 Hymenostomum. 



Fl. Fern, terminalis. 



Stoma edentuluin, clausum epiphragmate (e membraua exteriore orto), disco 

 tenuissimo (a columella libero) mox rupto et evanido ; Umbo persistenti hori- 

 zontal! indiviso. 



Calyptra dimidiata, lavis. 



Ft. Mas. terminalis, gemmiformis. 



