THE PALEONTOLOGIC RECORD 73 



Tropidoleptus carinatus, Cypricardella bellistriata, RMpidomella van- 

 uxemi, Spirifer marcyi and Delthyris mesacostalis (=D. consobrinus) 

 and others; and the Owego and Swartwood zones appear in the midst 

 of a characteristic Chemung fauna both above and below them. In the 

 Owego recurrent zone both Phacops rana and Dalmanites calliteles 

 occur. 



The Van Etten recurrent zone lies entirely below the range of 

 Spirifer disjunctus and associated species of the Chemung formation. 

 On following the sections eastward from the Waverly quadrangle the 

 species of the Chemung fauna become scarce, and east of the Chenango 

 Eiver very few species of the typical Chemung fauna have been detected 

 — although they are still abundant in the Chemung rocks to the south- 

 east and southward across Pennsylvania, Maryland and Virginia. 



§ 5. These facts have been interpreted as evidence not only of a 

 general shifting of faunas coincident with a rising of the land along 

 the eastern edge of the present continent, but of oscillation of condi- 

 tions and alternate occupation of the area by two sets of faunas coming 

 from opposite directions and temporarily living in abundance in the 

 area of central New York. 



§ 6. The lithologic changes in the sediments containing the different 

 faunas are not sufficient to account for the change in fauna. In quite 

 a number of sections there is no appreciable difference in lithologic 

 constitution between the strata which for a hundred feet thickness 

 have been filled with characteristic Chemung species and the imme- 

 diately following thin zone (of a foot or two) with scarcely a trace of 

 the Chemung species, but holding, in great number, species which if 

 found by themselves would be undisputed evidence of the Hamilton 

 formation. 



§ 7. It becomes necessary therefore to suppose that the controlling 

 cause determining the presence of one or other fauna is not the char- 

 acter of the bottom on which the sediments which preserved the fauna 

 were laid. We are thus led to conclude that the qualities of the ocean 

 water have determined the shifting or migration of the faunas. The 

 conditions to which the faunas were adjusted were evidently those of 

 depth, salinity or temperature of the waters in which the species lived ; 

 and their change of habitation was occasioned by change in the direc- 

 tion, path or extent of flow of oceanic currents. 



This leads us to consider the principles of migration as affecting 

 marine organisms. 



§ 8. Migration of Species and Shifting of Faunas. — Migration as 

 commonly applied in natural history means the movement of large 

 numbers of the same species from one place to another in a general 

 definite direction at more or less regular periodic times. So birds mi- 

 grate northward with the advance of warm weather; some fish migrate 



