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POPULAR SCIENCE REVIEW. 
officers of the survey under the direction of its chief, Dr. Hayden, who him- 
self commenced these laborious investigations more than twenty years ago. 
The descriptions of cretaceous fossils alone occupy 508 pages, in which about 
240 species or distinct varietal forms are described, mostly mollusca ; 63 dis- 
tinct forms are described from the “ Brackish- water and Lignite Beds,” 2 
from the “ Wind River Tertiary,” and 7 from the White River Tertiaries.” 
These indications of numbers, however, give no sort of indication of the 
amount of valuable palaeontological information contained in this volume ; 
they represent a certain number of species of more or less interest to the 
student of fossils ; but the remarks upon them, and especially upon the 
genera and their subdivisions are of the greatest importance to palaeontologists 
everywhere. 
In his u Introductory Remarks ” on the geology of the locality from 
which his fossils were derived, Professor Meek gives elaborate comparative 
tables of the various sections exposed, showing the equivalence of the beds 
in the different localities. The lowest cretaceous deposits everywhere rest 
upon palaeozoic and generally carboniferous rocks, and in all the sections 
here given are regarded as the equivalents of the Dakota group in the 
Nebraska section, generally containing, like the latter, numerous plant- 
remains. Above this group come in the Nebraska section, the Fort Benton 
group, and the Niobrara group, the whole attaining a thickness of about 
1,400 feet in the typical locality, and forming the lower cretaceous series, 
equivalent to our Gray Chalk and Upper Greensand (Turonian and Ceno- 
manian of D’Orbigny). Above these the upper series consists of two groups, 
the line of demarcation between which does not appear to be very strongly 
marked, — the Fort Pierre group with a thickness of 700 feet, and the Fox 
Hills group 500 feet thick. These are regarded as the equivalents of the 
upper chalk and Maestricht beds (Senonian, D’Orbigny). In Alabama and 
Mississippi, and everywhere east of the Mississippi river, the second and 
third, or Fort Benton and Niobrara groups appear to be absent, as also in 
the New Jersey section, which has served Professor Meek as a term of com- 
parison between the American and European cretaceous series ; and in New 
Mexico the cretaceous deposits, according’ to Dr. Newberry, fall into three 
great divisions, the lowest of which is regarded as equivalent to the Dakota 
group, the middle one to the two upper members of the lower series, and the 
uppermost to the whole of Professor Meek’s upper series. 
With regard to the true position of the Dakota group and its equivalents 
in other localities, which are so remarkable on account of the leaves of 
highly-organised dicotyledonous plants found in them, there seems still to 
be room for difference of opinion, and Professor Meek himself says, u The 
evidence respecting the exact part of the European cretaceous series to which 
the Dakota group belongs is not entirely satisfactory, the few animal 
remains yet known from it being mainly casts, and, so far as determined, 
not such forms as can be regarded as especially characteristic of any par- 
ticular horizon in the cretaceous of Europe. Up to this time we also know 
of no single species being common to it and any of the beds alone ; but then 
we, as yet, know comparatively only a few species of animal remains from 
this rock. One of these, however,” he adds, 11 belongs to the cretaceous 
genus LeptosoJen ) while the other shells are allied to cretaceous species, and 
