458 SHIMER. 



with those most abundant along the entire Massachusetts coast north 

 of Cape Cod today, shows that the cHmate of this region has become 

 somewhat colder since the time this earlier fauna flourished so abun- 

 dantly. This fauna, representatives of which are rare or altogether 

 wanting off our coast today, is now dominant off the coast of Virginia, 

 though it ranges from Cape Cod to Cape Hatteras. Of the sixty some 

 species noted in our list, about half no longer occur north of Cape Cod, 

 or only rarely in sheltered places, but find their perfect environment 

 farther south. Ganong (7) mentions nine such sheltered areas, includ- 

 ing the Gulf of St. Lawrence, Oak Bay, New Brunswick, Casco Bay, 

 Maine, and Massachusetts Bay. Between the retreat of the glaciers 

 from this coast and the present time a period must have occurred 

 during which these waters were as warm as those from Cape Cod to 

 Cape Hatteras today, and during which this Virginian fauna migrated 

 northward. 



4b. This was followed by a refrigeration of these northern waters 

 sufficient to prevent the breeding of many of the species except within 

 a few areas protected enough to raise the temperature of the air and 

 water sufficiently during the summer, or breeding season, for the 

 development of the young. (The adult can stand a much greater 

 degree of cold than the young). Though Massachusetts Bay is one 

 of the places in which the Virginian fauna has persisted longer than 

 upon the less protected coast, yet even in Back Bay the shells living 

 after the beginning of this colder period (4b) show a most remarkable 

 decrease in both the number of individuals and the number of species 

 of this southern fauna. Though such typical southern forms as the 

 oyster and Mulinia lateralis persisted into 4b yet the vast majority 

 of the Virginian fauna, including Venus mercenaria, Pecten gibbus 

 borealis, Lae^'icardium mortoni, Triforis nigrocinctus and \ itrinella, 

 had ceased to exist in the Back Bay region. 



If we may judge from the Back Bay sections, this change from a 

 warm water fauna to one characteristic of colder waters, was abrupt 

 and was due to a corresponding alteration in climate. It is not probable 

 that a refrigeration of the ocean waters alone could have made its 

 influences felt so very decidedly as far inland as Back Bay. It is 

 not likely, either, that all the dift'erences between the more fossiliferous 

 lower portion (4a) with its warm water fauna and the upper portion 

 (4b), with its few fossils indicative of colder water, are due to the 

 partial closure of Back Bay by the tidal building of Boston neck. 

 This partial closure, bringing about a reduction in tidal scour, would, 

 of course, cause a more rapid accumulation of sediment within the 



