538 
and clays, comprising a well marked geologic unit stratigraphi- 
cally. : 
The author does not think that the lowest portion of the 
Potomac (equivalent to the Purbeck beds of the ‘old world) have 
yet been found, and as the great angiospermous flora of the upper 
Potomac (Amboy clays, etc.) is not represented in England, a 
comparison of the English Wealden with any portion of our 
Cretaceous would be practically restricted to the middle and lower 
Potomac of the East as far as known, and to the Trinity and 
Kootanie of the West. In the table of distribution of the Wealden 
flora the range is extended geologically and geographically so as 
to include from the upper Jurassic to the middle Cretaceous in all 
parts of the world where the flora has been recognized, thus giving 
at once a complete list of the plants and a complete representation 
of their distribution vertically and laterally. 
From the comparisons thus made it is evident that our lower ~ 
Potomac flora has so much in common with that, of the Wealden 
that the strata which contain it must be regarded as equivalent, 
whether we eventually decide them to be Jurassic or Cretaceous. 
In any enumeration of this flora, cycads necessarily play an 
important part and cycadean trunks have received special attention 
from the author. Plate xcix. shows a group of twenty-one from the 
Purbeck beds of the Isle of Portland, England; plate c., fourteen 
from the Potomac formation of Maryland; plate ci., eight irom 
the lower Cretaceous of the Black Hills, and plates ciii. and civ., 
two species from the scaly clays of Italy. | 
The fossil forests of the Isles of Wight and Portland are also — 
described, and from the wood collected by the author sections 
were prepared from which two new species are described by Dr. 
F. H. Knowlton: Avaucarwzylon Wallacei and A. Webbit. : 
In discussing the Mesozoic of Portugal, tables of geologic a 
tribution of species for that country are given and interesting 
comparisons are made between the floras of certain localities and 
that of the Potomac. | a 
The portion of greatest interest to the botanist, however, MY 
be found under the chapter on Archetypal Angiosperms. It was 
long recognized by palaeobotanists that the highly developed > 
angiosperm flora of the Dakota Group, Atone beds, Amboy clay$+ _ 
