INTRODUCTION. xxiii 



found. Thus, we presume, Anictangium, (the foreign A. aqua- 

 ticum,) may be distinguished from Hedwigia, Pterogonium from 

 Weissia, Leucodon from Dicranum, Fabronia, an exotic genus, 

 from Orthotrichum, and above all, Hypnum from Bryum. 



Still it must be acknowledged, that even on these principles, 

 which may at first sight appear so clear, it will be difficult to 

 assign characters to some genera which seem gradually to pass 

 into each other. It is, for example, hard to pronounce if Gym- 

 nostomum microstomum,G.fasciculare, and G. Griffithianum really 

 possess what should be considered a peristome. It bears the closest 

 resemblance to that membranous ring which, in an early state, we 

 see on the mouth of the capsule of Weissia qffinis and W. trichodes ; 

 but in these two species it breaks into teeth at a more advanced 

 period. The peristome of Orthotrichum presents remarkable 

 anomalies ; sometimes the teeth are in a single row and only of 

 one kind, as O. anomalum ; in O. striatum the peristome is 

 clearly double, the narrow teeth, or ciliae, arising from an internal 

 membrane ; whereas in most of the other species which have 

 ciliary processes they arise on the side of the larger teeth. In 

 Dicranum, the teeth are subject to vary, and to border, on the 

 one hand, upon Trichosfomum, and on the other, upon Grimmia, 

 in which genus we find the teeth sometimes split. In Leshea it is 

 difficult sometimes to see the inner membrane rising above the 

 mouth of the capsule, and then the peristome precisely agrees 

 with that of Nechera, to which perhaps the genus ought to be 

 united. In those Mosses which make yearly shoots, these some- 

 times arise so near the point of insertion of the fructification, as 

 to make the fruitstalk appear lateral, which is especially the case 

 in the genus Bartramia. Even the calyptra of some^ Mosses 

 seems to be intermediate, having so slight a fissure that we are 

 doubtful which we should call that of Cinclidotus and of Splach- 

 num ; sometimes in Trichostomum, besides the short fissure at 

 the base, we see in T. microcarpon a single longitudinal cleft 



