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.
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