appreciable survival since he encountered so few of them in spring and early 

 summer sampling in Delaware Bay , In addition the second spawning of the 

 season is slight and probably does not materially augment the drill population. 



Growth 



Information on the rate of growth of U.- cinerea is grossly incomplete 

 particularly for' drills in North American waters. Likewise little is known about 

 the relationship between growth rate and such factors as kinds and quantity of 

 food, sexual activity, temperature ; salinity, turbulence, turbidity . and sub- 

 stratum . 



Height of shell (more popularly referred to as "length" of shell ), from 

 the apex of the spire to the tip of the siphonal canal, has been widely used in 

 reporting size data on drills , For young snails which grow relatively fast this 

 dimension has proved useful-, but for larger gastropods Stauber (1943) suggests 

 that volume is probably a better measure, since wearing of the shell may actually 

 indicate a decrease in shell height, In the case of large Uro salpinx a small in- 

 crease in shell height is attended by an appreciable increase in volume . 



The employment of shell size, either height or volume, as a criterion in 

 the determination of drill age is unsatisfactory because young drills hatch con- 

 tinuously throughout the warmer months of the year and thus provide a wide 

 range of sizes in each year class. Stauber (1943) in Delaware Bay repeatedly 

 obtained drills as small as 2 mm." in late July and August, and those only 4 mm , 

 in April which were probably .winter, survivors of the previous late summer ovi- 

 positions. T.-C. Nelson (1922) collected Urosalpinx, presumably hatchlings of 

 the previous summer, whose height ranged from 6 to 10 5 mm., average of 9 mm 

 in April in Little Egg Harbor : New Jersey, Considerable individual variation 

 also occurs in growth rate probably not only in the presence of variable food and 

 other ecologic conditions but because of individual genetic differences Stauber 

 reports that drills survived for 19 months in his laboratory without food; thus in 

 the field they could survive for long periods under poor food conditions without 

 an appreciable increase in height . 



A little information is available on the rate of growth of Urosalpinx in 

 America in the first year or two . In northern waters Pope (1910-11) and 

 Stauber (1943) report that on emergence from the egg case the young conch 

 varies 'in height Iroml^to-l". 5 mm7Tln"southerh waters Fedenghi (1931c) noted 

 that newly hatched drills aversge 0.8 to 1 mm. in height. Pope observed that 

 they double their size in 8 to 10 days in the laboratory. He assumed that since 

 the smallest drills found in early June measured 12.5 mm., this size represents 

 the growth of one year. JR. Nelson (1931) writes that drills collected in 



40 



