THE MIOCENE PERIOD 827 



had an influence in directing the courses of the coast currents. 

 Differences of climate in different latitudes had been developed 

 apparently, and cold and warm currents were probably more pro- 

 nounced than in earlier times, and their shiftings had still graver 

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

 northern shore tracts of the Atlantic and Pacific prevented their 

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

 tended further to intensify the provincial nature of the shallow- 

 water faunas. 



According to Ball, 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 vicissi- 

 tudes of the coastal tracts than most other forms, and whether 

 warm or cold waters prevailed, held their dominance. Figs. 552 

 and 553 show a few of the characteristic types. Compared with 

 the Eocene group, Fig. 535, the resemblances will be found 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, 1 probably due partly to intermigra- 

 tion and partly to parallel evolution. 



The marine fauna of the Pacific coast indicates a climate but 

 little warmer than that of the present, and this conclusion is re- 

 inforced by the plants of the Puget Sound region, which record 

 [transition from the subtropical climate of the Eocene to the tem- 

 [perate climate of the present. The fauna of the Upper Miocene 

 (indicates a still closer approach to the present. 2 



1 Md. Geol. Surv., Miocene volume, 1904, pp. 151-153. 



2 Arnold, Ralph, Jour. Geol., Vol. XVII. For Miocene marine fossils of 

 the Pacific coast, see Ball, Professional paper 59, U. S. Geol. Surv. 



suridea alticosta (Conrad); o, F. griscomi (Conrad); p, Xenophora con- 

 chyliophora (Born) ; q, Crepidula fornicata (Linne) ; r, Fulgar spiniger 

 (Conrad) var.; s. Ecphora quadricostata (Say); t, Siphonalia ma,rylandica 

 Martin; u, Ilyanassa (?) (Paranassa) porcina (Say). Scaphopod: 

 v, Dentalium attenuatum Say. (Maryland Geol. Surv.) 



