its growth and multiplication, and that it does not occur in any significant quantity 

 on these bottoms (Federighi, 1931c; Stauber, 1943; Adams, 1947; Cole, 1951). 

 Although Federighi states that the drill is unable to cross muddy areas, Adams 

 cites an example in Malagash Basin, Canada, where they traversed a barren mud 

 bottom about 75 feet wide and reached a dense concentration of oyster spat v/hich 

 were being reared in a dyked area, Engle (1935-36) in laboratory aquaria at tempera- 

 tures 8'C or below observed that drills moved at a rate of about 0.087 crn./min. 

 over sandy shelly gravel bottom, but only 0.063 cm./mir.. over mud bottom over- 

 laid with a few shells Stauber adds the observation that every several years in 

 Delaware Bay, especially on bottom muddied over for a. time, which probably 

 reduces the drill population, oyster spatfalls occurred which grew to market 

 size . 



It is also suggested that an unstable sandy bottom devoid of firm objects 

 is unfavorable for locomotion and possibly for survival (Federighi, 1931c; Cole, 

 1942; Stauber, 1943 ). Stauber, corroborating Cole, noticed that where sand pre- 

 dominates and shell or oysters are few, low densities of drills were encountered in 

 field trapping and dredging. The writer has observed in aquarium studies that 

 although drills remain on hard surfaces (clusters of oysters and sides of aquaria) 

 much of the time, they occasionally and quite readily creep over sand or mud from 

 one hard surface to another. 



Favorable substrata for LL cinerea seem to consist of lower intertidal and 

 subtidal surfaces of wood, metal, and rock, and of firm sand and mud or mixtures 

 of these overlaid by shell and/or living oysters, mussels, and other epifauna; and 

 combinations of these surfaces (Federighi, 1931c; Stauber, 1943; Adams, 1947; 

 Mistakidis, 1951; Cole, 1951; and others). 



Salinity 

 Minimum survival salinity 



U. cinerea frequently inhabit estuaries where seasonal fluctuations in 

 salinity resulting from rain fed floods of fresh water impose rigorous, even lethal, 

 conditions . This is particularly evident in the headwaters of estuaries where an 

 unseen oscillating barrier, the minimum survival salinity for the drill, inflicts a 

 rigid chemical restraint upon upbay distribution (T. C. Nelson, 1922; J. R. Nelson, 

 1931; Galtsoff et al., 1937; Stauber, 1943; Engle, 1953; Clancy, 1953) Careful 

 studies have shed some light on the response of the oyster drill to variations in 

 salinity in relation to such ecological factors as temperature and time . 



Federighi (1931c) choosing as the "salinity death point" that locus in his 

 data where approximately 50% of the drills in laboratory trials died after 10 days 

 of exposure to varying salinities at summer temperatures, concludes that drills 



6] 



