Maryland Geological Survey 333 



columbella, Exogyra Iwviuscula, Inoceramus exogyroides, Inoceramus 

 involutus, Inoceramus umbonatus, Mortoniceras vespertinum, Mortoni- 

 ceras shoshonense, and Baculites asper. 1 Some of these species, notably 

 the Inocerami, are so widespread and so sensitive that they have been 

 utilized as horizon markers in Europe as well as in America. 



The Mortoniceras subzone and the Niobrara are probably not taxonomic 

 equivalents, since in the Western Interior the Benton fauna in one area is 

 contemporaneous with that of the Niobrara in another; physical condi- 

 tions and the resulting faunas along the East Coast and in the Western 

 Interior during the Matawan were too dissimilar to admit of so exact a 

 correlation, but the molluscan evidence is consistently in favor of the 

 synchroneity of their characteristic faunas. In Maryland, not only the 

 ammonite fauna peculiar to the horizon disappeared at the close "of the 

 Mortoniceras subzone, but most of the cermalopods peculiar to the entire 

 formation. In the clearer waters of the Interior, however, they survived 

 until the close of the Pierre. 



Of the four remaining Matawan ammonites which have been reported 

 from Maryland, Baculites ovatus, Pachydiscus complexus, and Placenti- 

 ceras placenta are represented in the Pierre either by identical species or 

 forms so closely allied that they have been confused in the synonymies. 

 Scaphites hippocrepis, however, has no western analogue. On the other 

 hand, Mortoniceras is represented in the Benton by two species (shosho- 

 nense Meek and vermillionense Meek and Hayden) which Meek and 

 Stanton both consider as not improbably identical with Mortoniceras 

 tcxanus. 



In the Monmouth physical conditions were reversed ; there was a slight 

 deepening of the seas along the Atlantic coast which cut out the extensive 

 oyster banks and permitted a few of the ammonites to thrive. There is 

 no reason to believe, however, that the waters ever exceeded 50 fathoms in 

 depth. In the Western Interior, on the other hand, there was a decided 

 shallowing of the seas which greatly reduced the number of Inocerami 

 and other clear-water genera which flourished in the Pierre. The ammo- 



1 Stanton, T. W., 1893, Bull. U. S. Geol. Survey, 106, p. 48. 

 22 



