September and hatched the spring of that season measured 8 mm. in height, and 

 accepts this as the approximate growth during the first summer in Delaware Bay. 

 Adams (1947) in eastern Canada finds that drills grow rapidly and reach a height 

 of 13 to 19 mm. during the first growing season. He does not describe the method 

 by which this information was obtained. 



Stauber (19430 was able to obtain some data on the growth rate of older 

 drills caged with oysters in Delaware Bay from May 24 to August 8, 1939. During 

 this interval, 21 drills ranging in height from 11-15 mm. (means 13.4+0.8) 

 reached a height of 16-24 mm. ( 20.7+1.6); 13 drills survived. Another group 

 of drills 21-30 mm. (25.7+1 .6) in height attained a height of 23-30 mm. (26.3+1 .4) . 

 The increase iti height in the smaller drills was about 55% of the original height, 

 and it was slight in the larger drills . In an unusual case a female isolated from 

 April to October increased in shell height from 15.2 to 31.4 mm. The possible 

 inhibiting effect of confinement and crowding on rate of growth is a factor which 

 might well be considered in future growth studies . 



The most complete data available on the growth rate of U. cinerea was 

 obtained by Cole (1942) for drills in English waters. Cole's fine report presents 

 a careful analysis of growth in males and females based upon growth marks and 

 on size distribution curves. He noticed that these curves usually show a number 

 of closely approximated peaks which when correlated with growth mark* appear 

 to approximate annual growth increments . Clear growth marks may frequently be 

 seen on the tip of the shell bounding the siphonal canal and occur more closely 

 spaced after the first few years of life. On large shells these occur at 1 to 3 mm. 

 intervals. Since considerable overlapping of successive year groups probably 

 occurs which seriously impedes or entirely prevents the fixing of the position of 

 the peaks in the frequency curves, Cole made use wherever possible of the size 

 distribution curves by the freehand method advocated by Buchanan- -Wollaston and 

 Hodgson (1929) . 'His growth data represent Measurements of some 1, 700 drills 

 which were collected principally by hand picking in the intertidal zone of the River 

 Blackwater and the River Roach, Essex, during the warmer months of the year 

 over a period of three years . All drills visible during low water of spring tides 

 were included in each sample . In 1941 samples were dredged at THORthly intervals 

 in the River Blackwater . 



This information is summarized in Table 6. Cole found no substantial 

 number of males over 36 mm. or females over 39 mm. He confirms earlier 

 reports that females grow more quickly than males and reach a larger size, and 

 shows that in general U. cinerea reaches a greater average size in Britain than on 

 the Atlantic coast of North America . In laboratory checks on growth rate he reared 

 drills hatched in July on small oyster spat in a plunger jar at Conwjiy. By the end of 

 the first feeding period these drills reached a maximum height of 12 mm . , the mode 



41 



