r 
766 Recent Literature, [July, 
a connected view of a most interesting assemblage of fresh-water 
andbrackish-water mollusks, belonging for the most part to a 
transition period of great scientific interest, that between and con- 
necting the Cretaceous and Tertiary periods, z. e., the Laramie. Dr. 
White first takes up each family in systematic order and traces 
the history of its occurrence so far as it has been learned, from 
the earliest known appearance of any of its species within the 
present limits of North America until the present time, with a 
general discussion of certain questions suggested by the facts 
stated. 
The author claims that the Laramie group is a transitional one 
between the Cretaceous and Tertiary. ‘Neither the Laramie 
group nor any true geological equivalent of it is at present known 
anywhere except in Western North America. It there occupies 
or is found at various localities within a large region, the present 
known limits of which may be roughly stated as extending from 
Northern New Mexico on the south to the British possessions on 
the north, and from the vicinity of the Great Salt Lake on the 
west to a present known distance out upon the Great Plains of 
more than two hundred miles from the eastern base of the Rock 
mountains. It has been traced within the western boundary of 
both Kansas and Nebraska.” The history of this controverted 
group is then given, and for the benefit of the general reader a 
brief sketch of the evolution of the North American continent is 
“The continent in its present shape has been produced by = 
coalescence of two or more principal. portions, which were ele- 
continental area. The two principal portions of the continent pre- 
vious to the Cretaceous period were an eastern and western one 
respectively, and before the close of that period they were sepa- 
rated by a broad stretch of open sea. By the continued slow rise 
of the whole continental area this broad stretch of open sea 
tirely fresh at the close of the Laramie period. During thei f pi 
ately succeeding Eocene Tertiary epoch at least, the great | 
f Wester? 
mie and 
t of the 
the plateaus and the great systems of mountains O 
North America into the structure of which these Lara 
Eocene strata enter. Some portions of the western pat 
