580 Mr. Brown on Lyellia, Leptosiomum, and Buxbaumia. 



Better evidence on the same subject is afforded by Trichosto- 

 mum, Didymodon, and Leucodon, in all of which the thirty-two 

 teeth are distinct, though approximated in pairs ; by the sixteen 

 bifid teeth of Dicramim and Fissidens ; and by the like number of 

 teeth with a perforated axis in Trematodon, TVeissia nuda, Didy- 

 modon latifoUum, and several species of Grimmia. 



In all the genera having a double peristomium I believe the 

 pellucid axis more or less manifestly exists ; but in these genera 

 there is a great uniformity in the apparent number of teeth in the 

 outer peristomium; there being no instance of actual division iu 

 this series beyond sixteen, or of a further approximation, unless 

 in Orthotricluim, in several of whose species the approximation 

 or even union of the double teeth by pairs takes place, while in 

 a few others the sixteen teeth are slightly divided at the apex, and 

 in the whole genus the pellucid axis is remarkably distinct. 



The only exceptions to the actual division into thirty-two, or 

 the structure indicating that number, in the simple peristomium 

 of mosses, occur on the one hand in certain species of TVeissia, 

 perhaps in Encalypta and in Octoblepharum, in all of which, I be- 

 lieve, there is a reduction to sixteen : and on the other in Fohjtri- 

 <:Tiuin, where the number is frequently increased, varying in the 

 different species, and chiefly by multiples of sixteen, from thirty- 

 two to eighty. In this genus also, whatever the number may 

 be, the teeth never have a semipellucid, but rather an opake or 

 thickened axis, and no tendency to union or even approximation 

 is observable. The constant equidistance of the teeth in Poly- 

 trichum seems to be connected with its peculiar mode of disse- 

 mination ; for as this takes place through the interstices of the 

 teeth, and as complete separation of the seeds seems necessary on 

 account of their extreme minuteness, a reduction in number 

 and consequent increase of size of these apertures would proba- 

 bly. 



