Federighi (1931c) in a confirmation of Pope's work, found drill perforations in 

 both shells of oysters and on every portion of the valves. Seventy-three percent 

 of the holes were located over or near the place of muscle attachment- In 1919 

 T. C. Nelson (pers , com.) attached brass tags to 1,000 oysters m Little Egg 

 Harbor, New Jersey, through small holes drilled in the umbones by means of a 

 dentist hand drill applied with constant pressure . He found that the variation 

 in time necessary to drill through the umbones of different oysters varied from 

 a few seconds to five minutes, and interprets this as due principally to the varia- 

 tion in the amounts of prismatic shell and chalky shell present in the umbone. 

 He adds that shell crystals may be harder when attacked on the ends than from 

 the sides. Haskin (1935) using a hand drill and constant pressure found little or 

 no correlation between ease of penetrating an oyster shell and oyster size, and 

 that the oyster drill does not necessarily attack the thinnest shelled oysters nor 

 the weakest spot on the shell of a single oyster. 



Observations by Cole (1942) of oyster drills over 25 mm. in height con- 

 fined with M ytilus edulis in the River Blackwater, England, show that these 

 Urosalpinx drilled principally near the thin edge of the mussel shells. The 

 writer (1951) observed that Urosalpinx appeared to express no specificity of 

 location in drilling Vol sella demissa when caged with them in the field . 



The actual rate of drilling through shell (as contrasted to the rate of 

 destruction of prey) is probably dependent upon the size of the drill, the relative 

 quantities of prismatic and chalky shell present in the shell of the prey, the 

 temperature and salinity of the water, and other unknown factors (Engle ; .1940; 

 Federighi, 1931c; Galtsoff et ai . , 1937; T. C. Nelson, pers. com.). In 36 

 experiments in Hampton Roads, Virginia, Federighi demonstrated that the average 

 rate of drilling through oyster shell was approximately 0.4 mm. per day. Galtsoff 

 et al. and Engle record a rate of 0.5 mm. per day. Pope (Field, 1924) determined 

 that the time required by drills to perforate oysters .1.5 inches long is approx- 

 imately 2 days; 2.5 inches long, 4 days; 3.5 inches long, 6 days: and 4 inches 

 long or longer, 7 days Orton (1927) estimated that the average time taken for 

 Urosalpinx to drill through an English brood oyster 1-2 inches long was 5-6 days. 



The diameter of the hole drilled by Urosalpinx is related to the size of 

 the snail (Stauber, 1943) Because of the bevelled or tapered shape of the per- 

 foration it is larger in diameter at the surface of the shell of the prey than at 

 the internal surface In laboratory observations Stauber found that newly hatched 

 Urosalpinx 1 to 1 .5 mm . in height rasped holes ranging from 120 to 220u in outer 

 diameter. Drills averaging 25 mm. in height drilled holes averaging 1 .4 mm. 

 outside diameter and 0.78 mm, inside diameter; drills under 15 mm. produced 

 openings 0.8 mm. and 0.54 mm. in diameter respectively. 



52 



