79 
borne in mind that it has its centers of maximum as well as mini- 
mum development. In other words, it has its evolute and involute 
forms. In some instances these differ considerably, so much so 
that without the connecting forms they appear as distinct species. 
In fact they are not infrequently so Weetsn, especially by those 
systematists who see a “ new species” in the most trifling varia- 
tion and who do not care to dispel the illusion by the necessary 
investigation. 
In this revision I have endeavored to establish the commonly 
recognized species rather than to make newones. Without ascer- 
taing the precise limits of the former, it would be impossible to be 
sure of the latter. And as many of the 7huidiums are sterile in 
certain localities, diagnostic characters have been drawn largely 
from the vegetative system. The number and position of the 
papillae of the leaf cells, more than their size, have been found 
quite constant and therefore of value in discriminating closely 
related species. The paraphyllia, important from a generic stand- 
point, are less so from a specific, because of their variability. The 
cilia and perichetial bracts in these, as in nearly all mosses, are the 
parts most likely to be modified by changed conditions of growth. 
Types have been seen when possible; when not, as was too fre- 
quently the case, authentic specimens. These have been studied 
with so much care as to almost preclude the possibility of mistake. 
Those species admitted into the Manual as peculiar to North 
America have likewise been closely investigated with the following 
results: Zhuidium erectum Duby is Th. delicatulum, as is proved by 
the type and the author’s own correction; Hypnum calyptratum — 
Sulliv. is 7%. microphyllum, with abnormal capsules, due to para- 
sites; Zhuidium Allent Aust. is a dubious sterile form most proba- 
bly of 7%. delicatulum ; Hypnum remotifolium Grev. is not a Thu- 
idium, neither is it North American. Zhuidium lignicola Kindb., 
admitted in Macoun’s Catalogue of Canadian Musci, represents the 
northern variation of 7%. microphylium from which it seems impos- , 
sible to separate it on good grounds. Hypnum tamariscinum Hedw., 
although quite common in Europe, appears to go by default in — 
North America. The tripinnate ramification and the Sear 
apical cell of the branch-leaves serve to distinguish it from Th. 
aelicatulum. 
