Walter (1910) in a comprehensive study of the ratio of maximum shell 

 aperture to shell height in 30, 903 U. cinerea collected in four different local- 

 ities in the Woods Hole area over a period from .1898 to 1908 observed that as 

 drills in a given population grow larger this ratio diminishes regularly. He noted 

 this trend in collections taken at biweekly intervals during the summer, and also 

 in collections made successively over a period of years during the first week 

 in August In the course of 7 years of successive collecting he found that this 

 ratio fluctuated regularly and reached the highest average in 1902 . Walter 

 implies that this probably is a reflection of the yearly increase in size of the 

 majority of , the drills in the population of a given locality and that the high 

 average will repeat itself in future years as the cycle repeats itself He be- 

 lieves that this gradual change in form as the drill grows larger is related 

 to internal developmental changes occurring during the life history of each drill 

 and is independent of environment. 



Growth rate, temperature, and latitude 



Rate of growth in the oyster drill is strikingly influenced by temperature, 

 and it is very unlikely that size increases occur during the colder months of 

 the year (Stauber, 1943; Pope, 1910 -.11). The existence of mean size variations 

 in drill populations in different geographic regions, which at first suggested 

 that drills grow to a larger size in colder waters, led to a number cf studies. 

 Fedenghi (1931c) seeking to explain these geographic variations on the basis 

 of environmental differences alone, was the first to suggest that drills might 

 grow to a larger size in waters of lower temperatures since he (1931a) had 

 noticed that drills in North Carolina grow to an average size of 15 mm in height,, 

 and those in Virginia to 23 mm in water colder by a mean temperature of about 

 4°C, Fraser (1930-31) from a study of .1,000 drills in Essex, England, obtained 

 a mean height of 30 mm. Comparing his results with Fedenghi 's he states that 

 there does seem to be some temperature correlation, the means of approximately 

 11, 17, and 20 C C corresponding to the mean heights of 39, 23, and 15 mm. in 

 Essex, Virginia, and North Carolina respectively He admits that this correla- 

 tion may be only superficial, although there is no doubt that the drill grows to a 

 much larger size in England than in most American waters . The giant drills 

 averaging 44 50 mm , in height which grow on the eastern shore of Virginia (Bilker, 

 1951) are probably a different subspecies and should be considered in a separate 

 category. Not long after, Fedenghi (1931b) had opportunity to measure over a 

 thousand drills from Woods Hole in water colder than that in Virginia and North 

 Carolina and obtained a mean height of only 21 mm. He rightly points out that sinci 

 salinity and possibly other ecological factors vary among these areas it is problem 

 atical what factors influence drill size . 



44 



