of the group being approximately 10 mm. He suggests that they might be ex- 

 pected to reach a height of about 20 mm . by the following July, the end of their 

 first year. He found that samples of drills dredged during the summer contain 

 appreciable numbers of small drills which usually have a suggestion of a peak 

 around tins figure ... he was unable to collect adequate samples of the small 

 drills because the majority passed through the rings of die oyster dredge used in 

 sampling. 



Hancock (pers. com.) confirms Cole's results and believes his method 

 of analysis of growth rates is reliable . Hancock adds that in the planning and 

 evaluation of such studies a number of factors should be considered: (1) young 

 drills hatch throughout much of the summer, and since the largest growth incre- 

 ments are added in the first two years, there is probably a marked size variation 

 among individuals of any given population throughout the season; (2) in a propor- 

 tion of specimens a thickening of the lip of the shell takes place and growth 

 appears to cease; (3) populations less than three miles distance from each other 

 in the same river, and those from shallow and deep water in the same area of the 

 river, are characterized by quite different average and maximum sizes; thus 

 great care is necessary in sampling a limited area, and in the choice of methods 

 of sampling which are employed . 



From the incomplete information available on growth rates in Urosalpinx 

 it may be tentatively suggested that in America the drill may reach a height of 

 8 to 19 mm. in the first summer (J. R. Nelson, 1931; Adams, 1947). The unusual 

 growth of a single female drill in Delaware Bay from 15.2 to 31.4 mm. in one 

 summer (Stauber, 1943), coupled with the possible maximum rate of growth 

 during the first summer, indicates that in America the fastest growing females 

 may achieve a height of 3 1 mm . in two years . This is in marked contrast to 

 Cole's (1942) data which suggest that in England it takes about four or five years 

 for a female drill to attain this height . It is difficult to comprehend that drills 

 originating in America should exhibit such retarded growth rates in English 

 waters; the available data is probably too inadequate to permit such comparisons . 



As a quantitative controlled check on the growth studies of Cole and others, 

 and to provide accurate data on growth rate, size and age at sexual maturity, 

 maximum size, and longevity of Urosalpinx, it is urged that Urosalpinx be reared 

 in the laboratory from the egg stage to senescence in isolated running sea water 

 containers. This drill is easily cultured in the laboratory, and a number of 

 permanent marine laboratories now have facilities in which it would be possible, 

 if necessary, to maintain populations of drills for as long as 15 years . 



43 



