THE EOCENE PERIOD. 289 
_ These marine strata of the “ Lignitic formation ” are of special 
interest, as showing such a commingling of Cretaceous and 
Tertiary types of life, that it is impossible to draw any rigid 
line in this region between the Mesozoic and Kainozoic sys- 
tems. Thus the marine beds of the Lignitic series contain 
such characteristic Cretaceous forms as /zoceramus and Am- 
monites, along with a great number of Univalves of a distinctly 
Tertiary type (Cones, Cowries, &c.) Upon the whole, there- 
fore, we must regard this series of deposits as affording a kind 
of transition between the Cretaceous and the Eocene, holding 
in some respects a position which may be compared with that 
held by the Purbeck beds in Britain as regards the Jurassic 
and Cretaceous. 
The Middle Eocene of the United States is represented 
by the Clazborne and Jackson beds. The Clazborne series is 
extensively developed at Claiborne, Alabama, and consists of 
sands, clays, lignites, marls, and impure limestones, containing 
marine fossils along with numerous plant-remains. The /ack- 
son sertes is represented by lignitic clays and marls which occur 
at Jackson, Mississippi. Amongst the more remarkable fossils 
of this series are the teeth and bones of Cetaceans of the 
genus Zeuglodon. 
Strata of Upper Eocene age occur in North America at 
Vicksburg, Mississippi, and are known as the Vicksburg series. 
They consist of lignites, clays, marls, and limestones. Fresh- 
water deposits of Eocene age are also largely developed in 
parts of the Rocky Mountain region. The most remarkable 
fossils of these beds are Mammals, of which a large number of 
species have been already determined. 
LIFE OF THE EOCENE PERIOD. 
The fossils of the Eocene deposits are so numerous that 
nothing more can be attempted here than to give a brief and 
general sketch of the life of the period, special attention being 
directed to some of the more prominent and interesting types, 
amongst which—as throughout the Tertiary series—the Mam- 
mals hold the first place. It is not uncommon, indeed, to 
speak of the Tertiary period as a whole under the name of the 
““ Age of Mammals,” a title at least as well deserved as that of 
“* Age of Reptiles” applied to the Mesozoic, or ‘‘ Age of Mol- 
luscs” applied to the Palzeozoic epoch. 
As regards the A/ants of the Eocene, the chief point to be 
noticed is, that the conditions which had already set in with 
the commencement of the Upper Cretaceous, are here con- 
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