INTRODUCTION. V 



gree of importance is to be given to the lateral and ter- 

 minal situation of the fruitstalk ; by the aid of which 

 natural groups (and these last should never be lost sight 

 of, although in the present imperfect state of the science 

 they must occasionally yield to more precise artificial 

 characters,) are often found. Thus we presume Anic- 

 tangium (the foreign A. aquaticum) may be kept from 

 tfedwigm, Pterogomum from Weissia, Leucodon from 

 Dicranum, Fabronia, an exotic genus, from Orthotri* 

 chum, 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 Gymnostomum micro- 

 stomum, 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 affinis and W. trichodes ; but in these two 

 species it breaks into teeth in a more advanced state. 

 The peristome of Orthotrichum presents remarkable 

 anomalies ; sometimes the teeth are in a single row, 

 and only of one kind, as in O. anomalum; in O. stria- 

 turn the peristome is clearly double., the narrower teeth 

 or cilia arising from an internal membrane ; whereas in 

 most of the other species which have ciliary processes 

 they originate on the side of the larger teeth. In Di- 

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

 the one hand upon Trichostomum, and on the other 

 upon Grimmia, in which genus we find the teeth some- 

 times split. In Leskea it is difficult sometimes to see 

 the inner membrane rising above the mouth of the cap- 



