Reviews—Palcogeography and Diastrophism. 227 
quotes. Dr. Arber believed that the recently discovered genus 
Rhynia, founded on petrified specimens from Aberdeenshire, is 
identical with Dawson’s Psilophyton. It is certain that the two 
are very closely allied, but their generic identity has hardly been 
established. It is contended that Psilophyton and other Middle and 
Lower Devonian plants, despite the possession of a well-developed 
water -conducting strand of xylem, were Thallophyta and 
anatomically half-way between Thallophyta and Pteridophyta ; 
they are assigned to a new group, the Procormophyta. The much 
more highly differentiated Upper Devonian plants, grouped under 
Sphenopsida, Paleophyllades, Pteropsida, and Lycopsida, are 
believed to represent distinct lines of evolution from Algal ancestors ; 
the development of this view is an important part of Dr. Arber’s 
thesis. 
That Psilophyton and its associates were in some respects 
primitive is practically certain, but to regard them rather as 
Thallophytes than Pteridophytes is in effect to dispense with the 
recognized distinctions between these two classes. It may be that 
as we learn more about the oldest terrestrial plants we shall realize 
the futility of class distinctions based on existing types; class 
labels are, after all, of secondary importance in comparison with the 
wider question of the origin of the vascular plants, and it is from this 
broader standpoint of evolution that we should weigh the arguments 
presented by Dr. Arber. 
A. C. SEWARD. 
PALHOGEOGRAPHY AND DIASTROPHISM IN THE ATLANTIC—ARCTIC 
ReGcion DuRING Pat#ozoic Time. By Ovar HoLtTepadt. 
Amer. Journ. Sci., vol. xlix, No. 289. January, 1920. 
is studies of paleophysiography the Americans have been recently 
to the fore, and a summary of their work has appeared in 
Outlines of Geological History, with special reference to North America, 
by Bailey Willis & Rollin D. Salisbury, published in 1910. 
In the present paper Dr. Holtedahl carries this work into the 
Kuropean region which most nearly approaches the American 
continent, and discusses the distribution of land and sea and the 
diastrophic changes which have occurred from early Cambrian 
times to the end of the Carboniferous period. Incidentally he 
remarks that “it will certainly be hopeless for the advocates of 
permanency of the oceanic basins to apply the theory to the 
Norwegian sea ”’. ; 
The construction of paleophysiographical maps is a dangerous 
thing, and study of these maps often shows the striking dissimilarity 
of supposed ancient land-masses from any existing land-area. The 
narrow tract of land represented in this paper as extending from 
Scotland to Scandinavia in Ordovician times is an example of this. 
But the author has taken into account the character and distribution 
of the Paleozoic rocks of the area under consideration in far greater 
