42 
EQUISETACILE. 
The characters of the sheath and its teeth will not, I fear, fur- { 
nish us with a very safe guide, since I find modifications of the 
same structure obtaining in all the species hitherto described : | 
the ribs of the stem are arranged in proximate pairs, w^hich pass ■! 
into the sheaths, and one pair enters each tooth, generally ter- | 
minating in an obtuse somewhat rounded extremity ; the furrow 
between the ribs constituting the pair, is continued as a black j 
bristle, the margins of which are furnished with a whitish semi- | 
hyaline membrane ; this bristle, together with its marginal mem- 
brane, give an outline to the tooth. The diagrams below repre- | 
sent the outline thus given to the teeth in the three species: — 
the left hand figure represents E. hyemale ; the middle figure 
E. Mackaii ; and the right hand figure E, variegatum. In 
the variety Wilsoni the outline is somewhat more obtuse than in 
the last of these. I have selected for the figures examples in 
which the characters are strongly marked : I believe others may 
be found exactly intermediate. 
