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, gave rise to this very remarkable form in the class, allowed it to die out, and revived it again at various intervals?  This is by no means the only instance of the same kind.  There are a number of types making their appearance suddenly, lasting during one period or during a succession of periods, and then disappearing forever, while others, like the Rudistes, come in, vanish, and reappear at a later time.

[Illustration:  Rudistes.]