colonial period, low salinity not only constituted the barrier to drills that it does 

 today, but because of hydrographic conditions extent then, probably presented 

 a more stable check over a greater area than at present. This may be elucidated 

 as follows. In pre- and early colonial periods densely forested lands bounding 

 the coastal waters served to feed a relatively constant supply of fresh water to 

 estuaries, thus maintaining a wide relatively stable zone of brackish water the 

 year around suited to oysters but intolerable to drills . With the advent of the white 

 man and the consequent clearing of the land went much of the forest which had 

 slowed the flow of water back to the sea . Now, principally in the spring, rain, 

 melting snow and ice flood the estuaries at low temperatures at which reduced 

 salinities inflict relatively little harm to drills In the summer under conditions 

 of reduced fresh water flow salinity in estuaries mounts, permitting drills to 

 migrate farther upbay to previously uninfested oyster bottoms . With the invasion 

 of saltier water upbay, oysters also tend to set farther upstream, but in general as 

 estuanal shorelines converge toward fresh water the acreage of potential oyster 

 producing drill free gound is also reduced. 



[1 is questionable whether Urosalpinx exists in greater densities per unit 

 area today than in precolonial times, but the expansion of oyster culture onto 

 bottoms which did not previously support oysters, and the insidious invasion of 

 drills into these new areas and other areas with oysters, strongly affirms the ex- 

 istence of a total greater number of Urosalpinx t oday than in early colonial times, 



SUMMARY AND CONCLUSIONS 



1 The earliest fossil shells of U cinerea were collected in North 

 Carolina and in Maryland in Miocene deposits approximately 28 million years old. 

 The species is common along the Atlantic Coastal Plain of the United States in 

 Pleistocene deposits approximately one million years old in a range extending from 

 Florida to Massachusetts. 



2 . Man in oyster cultural practices has unintentionally accelerated the 

 mixing and dispersal of Urosalpinx so that today it is found broadly distributed 

 along the eastern coast of North America from Canada to Florida, along the west- 

 ern coast of North America from Canada to California, and on the eastern coast 

 of the British Isles. Its occurrence on the west coast of North America and in 

 Great Britain represents introductions by man Its centers of maximum density 

 appear to extend along the east coast of the United States from Chesapeake Bay to 

 Narragansett Bay Bathymetrically ir ranges from the mid intertddal zone to a depth 

 of at least 120 feei 



3, An anatomical and tunc tional d* s< i iption of the mantle cavity, nervous 

 system, circulatory system. Locomotory system, drilling and feeding organs, 



129 



