Maryland Geological Survey 73 



characteristic species of the Tinton and is confined to it. The abundant 

 presence of this form in Prince George's County cannot but sug- 

 gest a synchroneity with the New Jersey Tinton. It is difficult to 

 explain the absence of Belemnitella by any other than a stratigraphic 

 difference, since the conditions were apparently quite as favorable for its 

 existence in the later Monmouth as they were in the earlier. In the 

 European Mesozoic the Belemnitellas are considered among the most 

 valuable of the guide fossils since they originated abruptly, dispersed 

 rapidly and became extinct in as short a time as that required for their 

 initiation. It is equally difficult, however, to explain the absence of 

 B. americana by its early extinction, since its supposed European equiva- 

 lent, Belemnitella mucronata, is restricted to the upper portion of the 

 uppermost Senonian, a horizon higher than that generally accepted for 

 the Navesink. Aside from the presence of Belemnitella, the Bohemia 

 Creek fauna is notable for the relatively large number of Ostreids, a 

 feature which it shows in common with the later Matawan and the Nave- 

 sink of New Jersey. It differs from the Navesink, however, in the absence 

 of a large gastropod fauna. Apparently the waters were even more shallow 

 in the area inhabited by the Belemnitella fauna than in that character- 

 ized by the presence of Sphenodiscus and by 'the relatively few Ostreids, 

 particularly those of the more ponderous type. The Sphenodiscus fauna 

 is restricted in its known distribution in Maryland to the Western Shore, 

 and, indeed, to Prince George's County. These marls have furnished the 

 most prolific of any of the Upper Cretaceous faunas of Maryland. The 

 fossils are in an excellent state of preservation, though very soft and 

 prone to crumble. The characteristic elements of the fauna, aside from 

 the widespread Sphenodiscus, are Nucula slackiana, Cucullcea vulgaris, a 

 number of small oysters, Exogyra costata in limited numbers, Trigonia 

 eufalensis, a number of Pecten, notably simplicius and argillensis, Gre- 

 nella serica, Liopistha protexta, Crassatellites vadosa, several Cardia, the 

 prolific Cyprimeria major, two new species of Cymbophora, and a large 

 number of Corbulm, Pleurotomidce, Volutes, Pyrifusi and Naticidce, 

 together with Turritella in great abundance. The absence of Brachiopods 

 and Scaphopods is rather remarkable. 



