20 
EOCENE TEREBRATULA 
the most diagnostic fossils, often being present in great profusion. 
Three questions naturally arise: Did this species survive from 
the Cretacic times into the Eocene? Do the Eocene beds in which 
it occurs represent mechanically reworked Cretacic deposits? Is 
our form a new species? 
Survival from the Cretacic time to the Eocene period is highly 
improbable, since the majority of the members of the Cretacic 
fauna does not.^ It is difficult to conceive how this one form 
among so many could have survived during such a lapse of time 
as is certainly represented by the unconformity between the Cre¬ 
tacic and the Eocene strata, particularly since some of the asso¬ 
ciated forms were more hardy and more plastic if we may judge 
from their abundance and range. Moreover, at the outcrop where 
the principal Eocene collection was made, the Eocene deposits 
rest unconformably upon Late Cretacic sediments not younger 
than Aturian, if as young, and older than Rancocas; and this is 
not by any means the oldest Eocene formation known. This last 
fact also militates strongly against the possibility of the Eocene 
Terebratula having been reworked from older deposits containing 
Cretacic faunas. In addition the condition of preservation does 
not substantiate this view, as mechanically derived fossils always 
show some evidence of the wear incident to the erosion and re¬ 
working of the deposits in which they are found, it would in this 
case have been considerable if we may assume that all traces of the 
Rancocas formation were removed from Maryland west of Ches¬ 
apeake Bay. 
Some of the shells are slightly distorted; but no more so than 
the shells of the New Jersey and Delaware Rancocas formation. 
There are no evidences of transportation and what changes have 
taken place with regard to the external ornamentation is no more 
than what one would expect from shells ordinarily. The valves 
are filled with greensand, cemented by siliceous, calcareous and 
ferruginous materials, and are contained in a matrix of the same 
character of material, which fact, of course, does not prove any¬ 
thing ; but such evidence is not negative as it might be were the 
matrix of a different nature. 
Then, can this Terebratula be derived from the Cretacic sedi- 
2 On this point see L- W. Stephenson. U. S. Geol. Surv., Prof. Pap. 90, p. 157, 
1915. 
