52 ROYAL SOCIETY OF CANADA 
efforts to untangle the difficulty." Kraus? and Solms-Laubach,* among 
recent authorities, have done more than any others to give a clear and 
systematic conception of the real nature and relations of these plants. 
Knowlton has also summarised our knowledge of them in such a way 
as to afford a working basis of considerable value.* 
Looking at the various descriptions of the internal structure of 
the stem, one is immediately impressed with the inadequacy of the 
diagnoses and the too often loose and unscientific way in which the 
facts are stated, giving no adequate ground for comparison and the 
accurate differentiation of one species from another. Thus in D. New- 
berryi, one of the most important structural facts is entirely neglected 
in the description of the species, while in others, the mere statement 
that a certain detail is more or less like that of another species in which 
it is poorly or inaccurately described, conveys no accurate conception 
of the real facts, and forms no proper basis for comparison. 
Such looseness is a common fault and appears even in the most 
recent publications. The explanation is no doubt to be found in the 
fact that there has been no general revision of all the species on the 
basis of the stem structure, but one author has copied from another 
and in such copying there has been no direct reference to the original 
types for purposes of verification. 
The work of Sir William Dawson, extending over half a century, 
has resulted in the accumulation of a large amount of material from 
various parts of North America, the greater portion of which formed the 
basis of published descriptions, but there was also a considerable 
amount of material which had never been fully determined. This em- 
braced species described only in his notes, as well as others which he 
had not attempted to describe. There were thus some species without 
names, and others to which he had assigned specific names. Much of 
this material has been found to be identical with species already well 
known, while other portions embraced new species, and in such cases 
the names employed by Sir William have been retained and are now 
published for the first time. The accumulation of a particularly valu- 
able collection of type material, presented an opportunity for authorita- 
tive revision of the genus, which was not to be neglected. 
These considerations seemed to me sufficient to justify prosecution 
of the work as promptly and as completely as possible, and although it 
has now been brought to a conclusion, it is to be regretted that circum- 

1 Proc. U. S. Nat. Mus., 1890, XII., 601. 
2 Schimper’s Traité de Pal. Veg., II., 1870. 
8 Fossil Botany, 1891, 105. 
* Proc. U. S. Nat. Mus., 1890, XII., 601. 

