REVIEWS. 
81 
unlike jurassic forms. In addition to this, the modern affinities of the 
numerous leaves of the higher types of dicotyledonous trees found in it, 
present a strong objection to the adoption of the conclusion that it may 
belong to a lower horizon than the Upper Greensand of British geologists, 
while its position directly below beds almost beyond doubt representing the 
Lower or Gray Chalk, precludes its reference to any higher stratigraphical 
position.” Professor Meek consequently regards the Dakota group as the 
American equivalent in whole or in part of the European Upper Greensand. 
Overlying the Fox Hills group, the highest of the undoubted cretaceous 
series, come what the author calls the “ Fresh and Brackish-water Lignite 
Deposits,” a series of sands and clays, with interspersed beds of impure 
lignite. These deposits are divided into two groups — the Judith Biverand 
Fort Union groups. The latter, which is regarded as the more recent of 
the two, is also much more widely spread than the other over the Missouri 
region. The chief interest attaching to these deposits, however, and especi- 
ally to the older, or Judith Biver group, is to be found in the singular 
assemblage of fossil remains which they contain. This seems to mark them 
out as passage beds between the Cretaceous and Tertiary series. Thus the 
vertebrate remains found in them seem to point directly to Cretaceous 
affinities some of them being Dinosaurians related to Iguanodon, Megalo - 
saurus, &c .; whilst associated with these are others, such as a tortoise belong- 
ing to an Eocene genus, and garfishes of the genus Clastes, which, with the 
remains of plants also found in the deposits, would seem to indicate an 
Eocene Tertiary date. Similar, if not identical, deposits are recorded as 
occurring in various parts of the country, and especially in Southern 
Wyoming, where a lignitiferous group, called by the author the Bitter 
Creek series, exhibits in a very striking manner the intermixture of 
Cretaceous and Eocene types above referred to, but with this remarkable 
addition, that some species of shells obtained from low down in the series 
exhibit striking Cretaceous affinities, whilst others from a higher horizon 
appear to belong to decidedly Eocene types. On the whole, it would 
appear that in the Judith Biver group and its equivalents in other localities, 
we have distinct traces of the transition from Cretaceous to Eocene con- 
ditions. The Fort-Union group appears to be of Eocene age. 
The Wind Biver and White Biver groups, from which a few fossils are 
described in this volume, are of Miocene age. The former does not belong 
to the Upper Missouri region ; the latter is a series of sands, clays and 
grits, forming what are known as the Mauvaises Terres , or “ Bad Lands ” of 
Nebraska, and it is from these deposits, which may have a total thickness 
of about 1,000 feet, that the extraordinary Mammalian remains lately 
described by Professor Marsh have been chiefly derived. The Mollusca 
obtained from these deposits, which are so rich in the remains of Vertebrata, 
are but few — only seven are here described by Professor Meek, and all are 
terrestrial or freshwater species. 
Of the illustrations, which occupy 4o plates, we need only say that, as in 
all the publications of the American Surveys, they are abundant in quantity, 
and for the most part excellent in quality. In fact, in every respect 
Professor Meek’s work on the Cretaceous and Tertiary Invertebrata of the 
Upper Missouri is one of the finest and most complete contributions to the 
NEW SERIES, YOL. I. — NO. I. G 
