572 Mr. Brown on Lyellia, Leptostomum, and Duabaumia. 
Mr. Hooker, however, has since found, as he states, the same 
structure in several other mosses, generally considered as having 
a naked peristomium, particularly in Gymnostomum microstomum, 
Griffithianum, and fasciculare ; and as these species have but little 
resemblance to each other, and still less affinity to Leptostomum, 
he has reduced this genus also to Gymnostomum. 
If the statement referred to be correct, 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, Gymnostomum microstomum, 
the peristomium is certainly very different from that of Leptosto- 
mum. Jn this species I find, on removing the operculum, that the 
mouth of the capsule is not only completely covered by a hori- 
zotital 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 centrai 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 considered as 
a distinct genus, to which the name of Hymenostomum* may be © 
given; and it is worthy of remark, that in its technical character 
it approaches most nearly to Lyellia, though no two mosses dife 
fer more > in almost every other respect. 
* HYMENOSTOMUM. 
Fi. Fem. terminalis. 
Stoma edentulum, . clausum epiphragmate (e membrana exteriore orto), disco tenuissimo . 
(a columella libero) mox rupto et evanido ; lino persistenti horizontali indiviso. | 
Calyptra dimidiata, levis. 
Fl. Mas terminalis, gemmiformis. 
: From 
