THE GEOLOGICAL MIDDLE AGE. 
173 
Table of Periods in the Cretaceous Epoch. 
Maestrichtian . 
Senonian 
Turonian . . 
Cenomanian . 
Albian . . 
Aptian . . 
Ithodanian . 
Urgonian 
Neocomian . 
Valengian 
1 
Chalk. 
Chalk Marl. 
Green Sands. 
Wealden. 
One of the most peculiar and distinct of those 
unique types alluded to above is that of the Ru- 
distes, a singular Bivalve, in which the lower 
valve is very deep and conical, while the upper 
valve sets into it as into a cup. The subjoined 
wood-cut represents such a Bivalve. These Rudis- 
tes 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 num- 
bers in the Cenomanian, Turo- 
nian, and Senonian periods, 
and disappear again in the suc- 
ceeding one. These can hardly 
be missed from any negligence or oversight in the 
examination of these deposits, for they are by no 
