METHODS OF PALEOGEOGRAPHY 445 



numbers, because the collecting of fossils is carried on under conditions 

 far superior to those in force today when dredging for existing faunas. 



On comparing these figures with those for the Cincinnati region, proba- 

 bly the best known Paleozoic locality in America, it will be found that 

 from the Cincinnati series, which has a maximum thickness of about 850 

 feet, there have been gathered about 1,100 species, if the undescribed 

 forms represented in the various collections are included. This series is 

 now divided into 14 zones, each having an average thickness of about 60 

 feet, thus giving a fauna of about 80 species for each zone — a figure far 

 below the average in the faunas of the present oceans, the smallest num- 

 ber being 407 species. The Eichmond formation is the most fossiliferous, 

 with an estimated number of 500 species ; as there are 6 zones in this for- 

 mation, this will give but 83 species to each subdivision, the latter hav- 

 ing, according to Cumings, an average thickness of 60 feet. Further, in 

 the Cincinnati faunas all the known fossils are included, not the mol- 

 luscs alone. Along the Atlantic shore, from the Eio Grande to the Arctic 

 region, Dall lists 1,364 species of shelled molluscs within the 100-fathom 

 zone. This is a far larger representation, probably four times greater, 

 than that of the entire fauna of the American Trenton which has equal 

 latitudinal extent. 



Other and similar evidence could be offered, but it will suffice to close 

 this subject by adding that no paleontologist who has looked into the 

 present dispersal of faunas understands how currents of similar tempera- 

 ture can keep shallow-water faunas from intermingling. It was the cur- 

 rents and the equable temperature of the ancient seas that facilitated the 

 migration of the shallow-water life, and this is especially true of the larval 

 and adult animals living near the surface of the sea. Land barriers and 

 shallow-sea marshes, together with decided temperature and saline differ- 

 ences in the water, are the effective causes preventing the distribution of 

 faunas. Temperature and saline barriers, however, are comparatively 

 seldom effective in geologic time, but the land barriers are continually 

 occasioning the localization of faunas, and by their breaking down permit 

 the intermigration of the localized biota. The entire subject of sea- 

 currents should not be used to explain faunal differences, but for the 

 present should be laid aside. The first need is to establish paleogeogra- 

 phy, a result which has as yet not been attained. 



Conclusions. — In making the maps herewith presented the greatest 

 stress has been laid upon the distribution of faunas, both as to time and 

 space, as known to paleontologists. When synchronous faunas were found 

 to be different and a lapse of connecting strata occurred, these facts were 

 interpreted as meaning that a land barrier more or less complete kept the 



