XXVI IXTUODUCTIOX 



knowledge of their geological and geographical ranges, it becomes of 

 the very greatest value, since one whole class of important criteria for 

 the interpretation of the strata is thus made accessible. The present 

 report includes the results of such an exhaustive study of the fauna of 

 the Miocene of Maryland, embracing both a critical review of the species 

 described by previous authors, as well as the description of a large num- 

 ber of new forms. It is believed that a much more accurate idea of 

 the faunal characteristics, as well as of the physical conditions prevail- 

 ing during the ]\Iiocene period on the Middle Atlantic Coast, will result 

 from the methods pursued in this investigation. Certainly the data 

 for the comparison of the fauna with those of other areas will be 

 greatly increased. 



General Stratigraphig Eelations. 



Our knowledge of the Tertiary geology and paleontology of the 

 Middle Atlantic Slope has been largely augmented since the days of 

 Conrad and Rogers, yet few fields have afforded better opportunities in 

 recent years for continued investigations, since very divergent opinions 

 have prevailed and even to-day find expression in the different inter- 

 pretations of the data. 



Both the Eocene and the Miocene divisions of the Tertiary in this 

 area have broad surface exposures, and are represented by characteristic 

 sections along the leading waterways. Both are also highly fossilifer- 

 ous, although the Miocene shows a greater diversity of species than 

 does the Eocene. This difference, however, is not so great as one would 

 infer from a perusal of the literature, since a large number of Eocene 

 species, many of them very common, have been until recently unrecog- 

 nized, or at least unrecorded. 



A brief discussion of the general relations of the Coastal Plain de- 

 posits in the Middle Atlantic Slope is essential to a clear comprehen- 

 sion of the Miocene formations, and several pages will be devoted to this 

 aspect of the subject. 



The Coastal Plain consists geologically of a series of formations that 

 were deposited as moderately thin sheets, one above another, along the 

 eastern border of the crystalline belt, elsewhere referred to as the Pied- 

 mont Plateau. The coastal deposits are slightly inclined eastward, so 



