No. 5.] THE INTRODUCED LITTORINA. 255 



localities, of the same general proportions as the adults ? If 

 they are of different proportions, do they differ in a definite 

 way ? Is the difference greater in the American than in the 

 British shells ? 



To answer these questions it will be expedient to divide each 

 complement of shells into five groups: one group including all 

 shells which are less than i 5 mm. in height ; a second, a third, 

 and a fourth group containing shells ranging respectively from 

 1 6 to 17, 1 8 to 19, 20 to 21 mm. in height; and a fifth group 

 containing all over 21 mm. in height. 



On Chart I it will be noted that there are respectively 114, 

 308, 319, 183, 76 shells in the several groups, and the distri- 

 bution of the 1000 shells in the several groups indicates, other 

 things being equal, the general size of the shells of a particular 

 locality. Thus, if the shells in a particular locality are small, 

 the lower groups will contain a larger number. If the general 

 proportions of the shell remain the same through the successive 

 stages from infantilism to senility, the curves of distribution of 

 these several groups will lie directly over each other, and a line 

 drawn through their ideal means will be vertical. If the pro- 

 portions of the shells vary with age, then the line connecting 

 the five ideal means (the curve of growth) will bend, and its 

 trend and its irregularity will indicate the amount of change. 



The younger shells from Tenby, Wales (Chart I), are slightly 

 more ventricose than the adults, the curve of growth tending 

 towards the right at the lower part of its course and towards 

 the left at the upper part. The amount of variation, however, 

 is only i.i degree (91.5 --90.4= i.i). Even less variation is 

 exhibited by the South Kincardineshire shells, while the curve 

 of growth for the Humber District is almost a straight line, 

 varying only .3 of i^>. In all three cases the old shells are 

 less ventricose. 



The course of the curve of growth on the first American 

 chart (Chart IV) indicates that the American shells, at least 

 from this locality, are, in this third respect, more variable. 

 Though the older shells still exhibit greater elongation, the 

 curve of growth covers 2.9 degrees, an amplitude more than 

 twice as great as that covered by the most variable British 



