134 CRETACEOUS PAjLEONTOLOGY. 



Mt. Laurel-Navesink formations in New Jersey. It is not neces- 

 sary to assume that this faunal element in New Jersey immi- 

 grated directly from the regions in Europe where the Belemnitella 

 mucronata zone is known to occur, since the facts may be inter- 

 preted to mean that this Belemnitella fauna has migrated from 

 some unknown province into both the regions where it is now 

 known to occur so conspicuously, in America and in Europe. 



If the assumption is correct that the Belemnitella element of 

 Mount Laurel-Navesink fauna was an immigrant from the east, 

 possibly from Europe, it is strange that it did not spread down 

 the Atlantic coast and occupy a conspicuous place in the faunas 

 of the Gulf-border region. It is true that the Belemnitella does 

 occur rarely in the south, although it is never a conspicuous mem- 

 ber of the faunas, but Terebratella plicata is not known to occur 

 in that region at all. It is possible that the explanation of the 

 rarity of this element in the faunas of the South may be due to 

 the debouchure of a great Cretaceous river near the present posi- 

 tion of the Delaware or Chesapeake bays. A great stream of 

 fresh water might, under certain conditions, be a more or less 

 efficient barrier to the coastwise migration of certain forms of 

 shallow water life. To the squid-like Belemnites, however, such 

 a barrier would doubtless be less effective than to the brachiopods. 

 This explanation of the disti'ibution of the faunas is offered only 

 as a suggestion, since it is difficult to understand why such a 

 barrier should not have been just as effective against the spread 

 of certain conspicuous forms in the fauna which are evidently 

 of southern origin, as against organisms migrating in the oppo- 

 site direction, although the southern forms, so far as they are 

 recognized, are pelecypods and ' gastropods, while Terebratella 

 plicata, the one form which most clearly suggests a barrier of 

 some sort, is a brachiopod, a very different type of organism. 



Aside from this foreign element in the Mount Laurel-Navesink 

 fauna, we find recurring here the conspicuous Exogyra element 

 which had first appeared in New Jersey in the Marshalltown 

 fauna, although B. ponderosa of the earlier fauna has given place 

 here to B. costata. This species differs from the Belemnitella 

 and Terebratella, in being a conspicuous member of the Upper 



