THE MIOCENE PERIOD. 291 



tracts that were geographically continuous, though narrow, show 

 signs of having been cut into biological sections by special interrupting 

 agencies. Such barriers had perhaps been operative in certain pro- 

 vincial periods before, but they were not so well recorded as now. The 

 land area being large, great rivers joined the coast here and there and 

 poured volumes of fresh and muddy waters across the shore belt, doubt- 

 less forming barriers to some species, though probably not to others. 

 The warpings of the crust probably projected peninsulas and submarine 

 ridges out upon and perhaps across the continental shelf, and these 

 were not only barriers in themselves but supplemented their own influ- 

 ence by directing the courses of the coast currents. As differences 

 of climate in different latitudes had apparently been developed, cold 

 and warm currents were probably more active than in the previous 

 times of more uniform climate, and their shif tings had still graver 

 effects upon the faunas. So too, the lower temperatures in the northern 

 shore tracts of the Atlantic and Pacific shut off these tracts from serving 

 longer as migratory routes for the warm-water species, and this further 

 tended to intensify the provincial nature of the shallow-water faunas. 



According to Dall, 1 the Chesapeake Miocene was ushered in by 

 a marked faunal change due to a cold northern current driving out 

 or destroying the previous warm- water fauna of the region, and bringing 

 with it a cold-water fauna. There was a complete change of species, 

 and even some genera were displaced. The fauna retained, however, 

 a general molluscan aspect. Both the bivalves and the univalves 

 gave proof of better adaptability to the vicissitudes of the coastal 

 tracts than most other forms, and whether warm or cold waters pre- 

 vailed, held their dominance. Figs. 457 and 458 show a few of the 

 characteristic types. Compared with the Eocene group, Fig. 434, the 

 resemblances will be found, by the untechnical observer, more striking 

 than the differences. 



Notwithstanding the provincializing agencies, there were many 

 close correspondences between the faunas of the western and the eastern 

 sides of the Atlantic, probably due partly to intermigration and partly 

 to parallel evolution. These correspondences have been set forth by 

 Dall in the following quotation : 2 



" In a general comparison of the European and American Miocene we find, 



among other things which may be cited as parallelisms: in land vertebrates 



'Papers previously cited. 2 Md. Geol. Surv., Miocene volume, 1904, pp. cli-cliii. 



