ocomian, is accumulated along their base, while these in
their turn are slightly raised, and the Urgonian beds are accumulated
against them a little lower down. They follow each other from east to
west in a narrower area, just as the Azoic, Silurian, and Devonian
deposits follow each other from north to south in the northern part of
the United States. The Cretaceous deposits have been intimately studied
in various localities by different geologists, and are now subdivided
into at least ten, or it may be fifteen or sixteen distinct periods, as
they stand at present. This is, however, but the beginning of the work;
and the recent investigations of the French geologist, Coquand, indicate
that several of these periods at least are susceptible of further
subdivision. I present here a table enumerating the periods of the
Cretaceous epoch best known at present, in their sequence, because I
want to show how sharply and in how arbitrary a manner, if I may so
express it, new forms are introduced. The names are simply derived from
the localities, or from some circumstances connected with the locality
where each period has been studied.

_Table of Periods in the Cretaceous Epoch._

Maestrichtian } Chalk.
Senonian }

Turonian } Chalk Marl.
Cenomanian }

Albian }
Aptian } Green Sands.
Rhodanian }

Urgonian }
Neocomian } Wealden.
Valengian }

One of the most peculiar and distinct of those unique types alluded to
above is that of the Rudistes, a singular Bivalve, in which the lower
valve is very deep and conical, while the upper valve sets into to it as
into a cup. The subjoined woodcut represents such a Bivalve. These
Rudistes are found suddenly in the Urgonian deposits; there are none in
the two preceding sets of beds; they disappear in the three following
periods, and reappear again in great numbers in the Cenomanian,
Turonian, and Senonian periods, and disappear again in the succeeding
one. These can hardly be missed from any negligence or oversight in the
examination of these deposits, for they are by no means rare. They are
found always in great numbers, occupying crowded beds, like Oysters in
the present time. So numerous are they, where they occur at all, that
the deposits containing them are called by many naturalists the first,
second, third, and fourth _bank_ of Rudistes. Which of the ordinary
Bivalves, then