These preliminary data are much too incomplete and collected too variably 

 to permit the correlations which Fedenghi and Fraser attempted, though the varia 

 tions which Fedenghi encountered are not unexpected. There is reason to believe 

 that with Urosalpirx , as with other animalSj mutations and natural selection are 

 playing a role in the creation and perpetuation of characters which, even though 

 not conspicuous, will vary from one environment to another. 



Figures available on the sizes attained by the oyster drill in different 

 geographic regions are summarized in Table 7. These data further serve to 

 emphasize the range in variation in the sizes reported for this species, and thus 

 the difficulty at the present time of assigning average or maximal dimensions to 

 the species as a whcle. Table 7 tentatively indicates that three different drill 

 sizes, based on the maximum heights reported, exist: the giant American form of 

 maximum height 51-61 mm., the English form, 39-43 mm., and the small wide- 

 spread American form, 26-40 mm, whose height range overlaps that of the 

 English form. The average maximum height for the small American form is 

 approximately 33 mm. It should be stressed that to be most useful and meaning- 

 ful records of drill size should be based on both age and sex groups for each geo- 

 graphic region . 



Growth rate and quality of food 



Engle (1942) under controlled laboratory conditions in Long Island Sound 

 demonstrated that considerable variation in the rate of growth occurred among 

 four different groups of drills each of which was fed exclusively on an excess of 

 one of the following living animals: oyster ( Crassostrea virginica) , soft clam ( Mya 

 arenaria) , edible mussel ( Mytilus edulis) ; and barnacle (T&lanu s sp.). Observa- 

 tions were extended for 13 months, although feeding occurred only between May and 

 November . In the course of the feeding season Uro salpinx grew fastest on a diet of 

 soft clam, less on oyster ; even less on barnacle, and least on mussel. Older 

 stages of the soft clam are not ordinarily available to the drill in nature because 

 they are buried in the bottom . When exposed, soft clarns are more vulnerable to 

 attack by drills than the other food organisms because of the exposed soft parts, 

 and the fact that drills grew fastest on this diet, although it may reflect a more 

 nutritious diet, may also suggest a more accessible food. Engle further noted that 

 the maximum rate of growth on each of these foods occurred at different periods 

 during the deason: drills feeding on mussels grew most from June 12 to July 12; 

 on soft clams, from July 12 to August 10; and on oysters and barnacles, from August 

 10 to September 6. This seasonal variation in growth may be associated with a 

 parallel variation in the nutritive value of these food organisms . Engle observed 

 that on the mussel bed originally inhabited by the experimental drills only a few 

 drilled mussels were encountered, while oysters and barnacles there were attacked 

 in large numbers . 



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