239] J. A. Gardner 41 



from the Calvert of Virginia, approximately 12l/ 2 % of the 

 entire fauna, persist into the Eecent and furnish consistent 

 evidence of environmental conditions during Calvert Times. 

 The depth of the waters in which they live did not, in all 

 probability, exceed 20 or 25 fathoms. The temperature was 

 perceptibly lower than that of any other of the middle or 

 late Tertiary faunas of that region. The bottom was prob- 

 ably soft, dominaritly mud,, with a mixture of sand. At 

 least a portion of the shore must have been sufficiently shel- 

 tered to encourage the growth of kelp and ,sea lettuce and 

 other sea weeds to which many of the smaller univalves and 

 bivalves characteristically attach themselves. The Calvert 

 of Maryland is unusually varied for the latitude. It is quite 

 possible that the ancient shore line in that area was fringed 

 with islands and sand spits similar to those along the outer 

 margin of Virginia and North Carolina today and that dur- 

 ing Calvert times the spits were now washed away, admitting 

 the off-shore fauna, and now built up, protecting the waters 

 behind them and allowing a warmer water element to creep 

 in and establish itself. 



There is no evidence of any marked change in the ecology 

 in passing from the Calvert to the St. Mary's. All of the 

 recent species represented in the Calvert are present in the 

 St. Mary's but the number is almost tripled. The northern 

 element, however, is slightly less prominent and the southern 

 element a little more so. The fauna is prolific in individuals 

 but not greatly diversified. The outstanding differences be- 

 tween the St. Mary's molluscs of Maryland and those of 

 Virginia and North Carolina are mainly those of latitude, 

 although the presence in Maryland of a considerable number 

 of Surculas, one of the characteristically deep-water pleuroto- 

 mids suggests deeper water in that area. The faunas in Vir- 

 ginia and North Carolina are remarkably uniform. There 

 are, to be sure, a few species common to Maryland and 

 northern Virginia which are not found in North Carolina 

 and a few of southern affinities which are restricted to North 

 Carolina. The monotony of the assemblage indicates a long 



