AI.KINS : GROWTH AN'H VARIATIOX OK tINIO PICTORUM. 



The area enclosed between the lines shewing the upper and lower 

 limits, divided by the difference between the greatest and least lengths 

 found, gives a measure of the variability of the species. 



Table III. 



Length 



No. of 



Width millimetres. 



Thickness millimetres . 



mm. 



shells. 



Min. 



Max. 



Mean. 



Min. 



Max. 



Mean. 



15 



I 



32 



32 



32-0 



10 



10 



TO'O 



16 

















17 

















18 



3 



40 



43 



417 



1 1 



12 



11-7 



19 



4 



40 



42 



4i'25 



II 



13 



1 1 75 



20 



2 



44 



44 



44-0 



12 



13 



12-5 



21 



3 



46 



48 



47-0 



13 



14 



137 



2-2 



4 



48 



52 



49"25 



13 



15 



1375 



23 



22 



49 



56 



52-3 



13 



16 



143 



24 



8 



49 



57 



53"5 



14 • 



17 



i5'i 



25 



15 



53 



62 



57-1 



15 



17 



^5-5 



26 



25 



53 



64 



58-8 



15 



19 



i6-8 



27 



27 



57 



69 



6i"9 



15 



19 



16-9 



28 



17 



61 



71 



64-8 



16 



19 



i8-o 



29 



17 



62 



71 



67-4 



18 



21 



19-1 



30 



19 



64 



75 



69-8 



18 



22 



198 



31 



21 



69 



79 



73'9 



19 



23 



20'7 



32 



18 



70 



80 



74-6 



20 



24 



2 I "6 



33 



15 



73 



84 



77-0 



21 



25 



22-5 



34 



^7 



74 



84 



79-2 



22 



25 



23-0 



35 



9 



75 



87 



80-9 



23 



26 



24-5 



36 



2 



79 



84 



8i-5 



23 



25 



24'0 



37 



I 



88 



88 



88-0 



27 



27 



2 7'0 



Fig. 3 shews that the width is throughout a linear function of the 

 length — in other words, a definite increment in length is always 

 accompanied by an equally definite increase in the width. But in the 

 case of the thickness (fig. 4) this is no longer true : the ratio thick- 

 ness : length increases gradually as the length increases, and this 

 tendency becomes more pronounced as the length increases. 



In the case of the width : length relationship, the ontogeny of a 

 number of individuals was followed by tracing on the shell a series 

 of growth lines, and determining the length and width corresponding 

 to each of these. This was done for twelve shells, selected to give 

 as wide a range as possible in the value of the width : length ratio of 

 the adult shell. In every case the curve of width against length was, 

 within the limits of experimental error, a straight line. The figures 



