INTRODUCTION TO CRYPTOGAMIC BOTANY. 481 



certain whether the same species may not sometimes exhibit 

 the characters of the two genera. But however this may be, 

 the peristome is very variable in the same species, insomuch 

 that in different conditions it might be referred to several 

 genera. Mr. Wilson, for instance, mentions a form of Aiiaca- 

 lypta lanceolata, found at Bristol by Mr. Thwaites, with a 

 red peristome, the teeth of which were distinct almost to the 

 base, unusually long, with sometimes two rows of perforations. 

 The gymnostomous Weissiei are distinguished by the compact 

 reticulation of the leaves. The species are all monoecious, and 

 consequently abound in fruit. The species belong mostly to 

 the northern hemisphere, and to the colder parts ; but Pottia 

 Heimii occurs at the Cape, and is widely distributed in higher 

 latitudes ; and P. fiavipes in Chili ; while P. Leprieurii is 

 found in Cayenne. Not a single species occurs in New Zealand. 



6. Trichostomei, Br. S Sc, Mont. 



Peristome consisting of thirty-two filiform teeth, distinct or 

 united at the base, frequently approximating in pairs, and 

 sometimes spirally twisted. 



530. We have here, again, a large and very important mass 

 of species comprised under numerous genera, and exhibiting 

 various modifications of the peristome, which is, however, 

 always finely divided, and in one section curiously twisted, the 

 tissue of the operculum itself following the same course. All 

 agree more or less in the tufted habit, except where the indi- 

 vidual plants are scattered, as in some of the more obscure 

 species. Tortula ruralis is one of the most conspicuous of 

 our mosses, from the large patches which it forms on slated 

 or thatched roofs. Trichostomum glaucescens is remarkable 

 for the glaucous hue which occurs again in Bm'tramia. Des- 

 raatodon nervosus is sometimes destitute of peristome, while 

 at other times the slender tijDS of the teeth are twisted. It 

 occurs in either hemisphere. The tribe is eminently European 

 and American, but the tropics are not without species of either 

 section, and many occur in the southern hemisphere, which 

 are mostly distinct. Ceratodon oscillates between this tribe 

 and Dicranei, to the latter of which it approaches nearest in 

 the foliage, to the former in the peristome. Didymodon 

 31 



