untouched. Even drills starved for almost three months spurned them. When 

 these same drills were offered a shucked Anomia, they consumed it within 17 

 hours, Engle (1942) reports a similar occurrence. These investigators con- 

 clude that Anomia shell affords a barrier to Uro salpinx . Loosanoff (pers com.) 

 finds that Anomia can be drilled by Uro salpinx but only under abnormal conditions 

 when starved drills display interest even in forms which they normally do not attack 

 An occasional drilled Anomia valve has been found in the field (JR. Nelson, pers 

 com.; Andrews, pers, com.) . In the early fall of 1953 the writer placed 100 adult 

 drills on a square foot of bottom in a salt pond in Gardiner s Island. New York, 

 among oysters of various ages and 12 Anomia simplex about one inch in diameter . 

 These were confined under a screen cage. In late November it was found that 

 one Anomia was partially drilled, and two others were completely perforated, 

 but all three were living. Microscopic examination of the latter showed that the 

 drills had rasped away some of the flesh but that Anomia had regenerated the 

 mantle and secreted a thin layer of shell material over the perforation . Only one 

 instance of unusual predation of Anomia by the drill has ever been reported 

 (Glancy, 1954). This took place in a dense population of large sea stars ( Astenas 

 sp.), oyster drills, and jingle shells in Pecomc Bay. Long Island. No explana- 

 tion for this unusual behavior is available. These incomplete observations 

 suggest that the shell material and possibly the flesh of living A nomia may possess 

 a quality which in the majority of cases tends to repel oyster drills 



Drilling behavior 



Pope (1910-11). Federighi (1931c), and Stauber (1943) report that in the 

 field the right or flat valve of the oyster is usually perforated since this is 

 generally uppermost, but that in dense clusters or when oysters are resting on 

 their sides drilling may occur through either valve. Since in native surroundings 

 the left valve of young oysters is usually cemented directly to the substratum and 

 older dislodged single oysters tend to lie on the left valve on the bottom the right 

 valve is vulnerable to attack . Federighi reports that Uro salpmx generally chooses 

 the uppermost valve of an oyster for drilling even in oysters set m aquaria with 

 both valves exposed. 



In a series of detailed plottings of the distribution of the perforations by 

 Urosalpinx in the shells of oysters collected in the field arid in the laboratory, 

 Pope (1910-11) discovered that perforations are universally distributed over the 

 entire surface of the shell, and that for the most part the middle areas of the 

 valves are most frequently the site of drilling. He found no evidence to indicate , 

 as had been suggested by earlier observers, that perforations are confined to 

 the limits of the adductor muscle of the oyster, or that the drill always selects 

 depressions or the thinnest portion of the shells of its prey for attack . 



51 



