56 The Upper Cretaceous Deposits of Maryland 



by Margarites which was probably initiated before the opening of the 

 Cretaceous and still persists. The ancient family of the Euomphalidce 

 includes a species which has been rather dubiously referred to Discohelix, 

 a genus which is particularly characteristic of the Lias, although it has 

 been reported from the Trias to the Oligocene. The occurrence of the 

 single scaphopod is without significance. 



Only Eutrephoceras among the cephalopods survived the close of the 

 Mesozoic and that genus only into the Tertiary. All of the Ammonoids — 

 Pachydiscus, Baculites, Scaphites, Spheno discus, Placenticeras and Mor- 

 toniceras— are restricted in their distribution to the Cretaceous, while the 

 Dibranch Belemnitella has not been recognized excepting from the Upper 

 Cretaceous. 



The Raritan Formation 



Name and Synonymy. — The Earitan formation, so named by the 

 writer * from the Raritan Eiver, New Jersey, in the basin of which it is 

 typically developed, was later applied to deposits of the same age in Dela- 

 ware and Maryland. 2 The term Plastic or Amboy Clays had hitherto been 

 employed for this formation in New Jersey. Uhler included much of the 

 Earitan in his Albirupean formation which, however, also embraced por- 

 tions of the Patapsco and Patuxent formations in both Maryland and 

 Virginia. McGee at the same time apparently included portions of the 

 Earitan in his Potomac formation, although much of the Earitan both 

 in Maryland as well as farther north was not included. Ward and other 

 writers endeavored later to place all of the Earitan deposits in the Potomac 

 group with which, however, they should not be combined either on strati- 

 graphic or paleontologic grounds. The term Potomac group is therefore 

 employed only for the Patuxent, Arundel, and Patapsco formations of 

 Lower Cretaceous age. 



Areal Distribution. — The Earitan formation extends across the 

 state in a constantly narrowing belt from the Delaware line to the Potomac 



1 Clark, Wm. Bullock, Ann. Rept. of the State Geologist of New Jersey for 

 the year 1892, pp. 181-186, 1893. 



2 Clark, Wm. Bullock, Bull. Geol. Soc. Amer., vol. vi, p. 480, 1894. Clark and 

 Bibbins, Jour. Geol., vol. v, pp. 492-494, 1897. 



