AND BUXBAUMIA. 349 



In all the genera having a double peristomiura 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 in this series beyond sixteen, or of a 

 further approximation, unless in Orthotrichum, 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 shghtly 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 peri- 

 stomium of Mosses, occur on the one hand in certain species 

 of Weissia, perhaps in Enccdypta and in Odoblepharum, in 

 all of which, I believe, there is a reduction to sixteen : and 

 on the other in PolytricJium, 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 observ- 

 able. The constant equidistance of the teeth of Foly- 

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

 semination ; 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 reduc- 

 tion in number and consequent increase of size of these 

 apertures would probably, in some degree, prevent dis- [ssi 

 persion, while the unequal distances of the teeth might 

 either produce a deviation from the regular figure, or an 

 early rupture, of the tympanum, which forms an essential 

 part in this economy. 



BUXBAUMIA. 



In my former paper^ I have proposed to preserve the genus 

 Buxbaumia, as established by Schmidel ; and in construct- 



' lVol.i,p. 351.] 



