824 



to pure water can flourish close to the continental shore, and 

 (le])osits of a purely organic nature, such as the extensive 

 deposit of pteropod shells constituting the Styliolina lime- 

 stone in Western New York, may form in comparatively 

 shallow w^ater. 



The epicontinental seas are es])e(*ially adapted for the 

 development of local or i)rovincial faunas. Such provin- 

 cializing of faunas is most marked if, Iw some oscillatory 

 movement of the land or some other physical change, the 

 basin of the eincontinental sea should become separated from 

 the exti'a-continental portion of the littoral district suffi- 

 ciently to prevent intercommunication between the organ- 

 isms of the two provinces. A bari-ier is thus formed, which 

 need not necessarily be a land bari'ier, and a great diversity 

 of faunas may result. Such diversity of fauna existed in 

 early Tertiary time between the Mississippi embayment and 

 the Atlantic coast ; and in Palfpozoic time, between the Bay 

 of New Yoi-k and the Central Intei-ior sea. Recent provincial 

 faunas are frequently met with. It requires oidy a com])ara- 

 tively slight elevation of the sea flooi", or a moderate dee])en- 

 ing of the abyssal oceanic l)asins, to draw off the water from 

 the shallower regions, and lay large portions of the littoral 

 district dry. Such a change would, of course, result in an 

 extinction of the whole of the littoral flora and fauna thus 

 exposed, and force the survivors to accommodate themselves 

 to a narrower field. Revival of stream activities, conse- 

 quent upon elevation of the land, would result in carrying a 

 large amount of debris into the sea, and thus produce con- 

 ditions unfavorable to the existence of many organisms. 

 Such an elevation of the land and extinction of faunas oc- 

 curred at the close of Orclovicic time* in the area of the 

 central and eastern interior PakTeozoic sea. A few survivors 

 only, of the Ordovician fauna, occur in some of the lower 

 detrital beds of the next succeeding formation, in certain 

 localities. In the Siluric era a new fauna developed, as 

 conditions again became favorable. 



* Weller, '98, p. 603. 



