42 Environment of Tertiary Marine Faunas [240 



stretch of open shore with only an occasional bight from the 

 vicinity of the present York River in Virginia to that of 

 the Neuse River in North Carolina. The slope of the conti- 

 nental shelf must have been very gentle, not more than 3' to 

 the mile, since there is no perceptible change in the bathy- 

 metric character of the fauna between the extreme eastern 

 and western outcrops, a distance of 60 or 70 miles. There 

 is no reason to believe that any part of this platform was 

 submerged to a depth of more than 30 or 40 fathoms. The 

 bottom was doubtless soft and, for the most part, muddy 

 since the mud-burrowers, notably Mulinia are exceedingly 

 prolific and widely distributed. The waters must have been 

 sufficiently clear, however, and the bottom sufficiently shelly 

 to furnish clutch for the numerous oyster spat and to permit 

 them to mature. Conditions were probably not very favor- 

 able to algal growth, since most of the groups which charac- 

 teristically attach themselves to the sea-weeds of various 

 kinds have a meagre representation. 



The elevation along the Hatteras axis at the close of the 

 St. Mary's was apparently great enough to cut off the York- 

 town basin in Virginia from the Duplin in southern North 

 Carolina. The faunas of the two basins, though similar in 

 general character, differ more in detail than one would expect 

 in two shallow water faunas only a couple of hundred miles 

 apart. The contemporaneity of the Yorktown and Duplin 

 faunas was suggested by 'Dr. Dall more than fifteen years ago 

 and even at that time he brought forward in explanantion of 

 the conspicuous faunal differences the potency of the ocean 

 currents, a factor which has been so emphasized of late in the 

 distributional studies upon the West Coast. The Yorktown 

 fauna is strikingly like that listed by Bartsch and Henderson 

 from Chincoteague Bay, Accomac County, Virginia. The 

 greatest break in the East Coast life from the late Tertiary on 

 to the Recent comes at Hatteras, the point at which the Gulf 

 stream leaves the inshore and swings out toward the open 

 sea. Many of the sub-tropical species are able to follow along 

 the shore as far as it is protected by the warm current, which 



