FilRNS OF GREAT BRITAIN. 151 



to the plant. By some chemical process, the silica may 

 be wholly freed from the vegetable portion, and the 

 entire form of the stem and branches of the Horsetails 

 preserved in beautiful clear crystal ; and when the vege- 

 table remains are washed after the process they are 

 found to be quite free from a single particle of flint. The 

 sheaths of this species clasp the stem quite closely, and 

 are marked, though less strongly, with the same number 

 of ridges. Black membranous bristle-shaped teeth, equal 

 also in number to the ridges, terminate the sheath, soon 

 disappearing, and leaving its margin indented with 

 roundish notches, though the teeth of the sheath just 

 beneath the cone remain. The teeth, which are at first 

 pale green, become afterwards black; they are pale in 

 the middle, and have a deep black ring both at the top 

 and base of the sheath. 



The catkin of this plant is small, and of a dark 

 colour, and usually terminates the deep green stem ; or, 

 if placed at the side, is never at any great distance from 

 its summit. The scales, which are from forty to fifty in 

 number, are marked with two or three lines. 



This is not a common species, and is apparently very 

 local in those counties in which it occurs, while it is 

 almost unknown in the midland and southern parts of 

 England. It has been found at Hawthorndean, Dur- 

 ham ; in the neighbourhood of Newcastle ; ia Cumber- 

 land and Westmoreland; near Scarborough, in York- 

 shire, and several other northern localities; also in 

 South Kent, and in several places in Ireland, Wales, 

 and Scotland. It is common in many moist lands and 

 woods in some continental countries, as in Germany 



