8 



W. E. Alkins — Variation of Sphceria 



TABLE V. 



Width 

 Length 



Ratio Distribution. 



TABLE VI. 



Thickness 



Length 



Ratio Distribution. 



Ratio : 



Number Ratio 



Number 



w 



of 



w 



of 



L 



Specimens 



L 



Specimens 



1-13 



1 



1-22 



19 



114 



6 



1-23 



14 



1-15 



9 



124 



6 



1-16 



13 



1-25 



5 



117 



13 



1-26 



1 



118 



20 



1-27 







119 



28 



1-28 



1 



1-20 



40 



1.29 



1 



1-21 



23 







Mean: 11964 





Standard Deviation : 



02761 



Coefficient of Variation 



: 2 31 



Ratio : 



Number 



Ratio 



Number 



T 



of 



T 



of 



L 



Specimens 



L 



Specimens 



0-57 



1 



66 



42 



0-58 







0-67 



34 



0-59 



2 



0-68 



22 



0-60 



2 



0-69 



15 



0'61 



1 



0-70 



7 



0-62 



9 



0-71 



2 



0-63 



17 



0-72 



3 



0-64 



19 



0-73 







0-65 



22 



0'74 



2 



Mean • 



6604 





Standard Dev 



iation : 0"02575 



Coefficient of Variation 



: 390 



It 



/a r i *• i 4i, d 4- Width , Thickness 



(/) Correlation of the Ratios i r and -y r, — 



v/ Length Length ' 



is of considerable interest to enquire whether there is any 



marked correlation between the two ratios we have been 



considering, i.e. whether there is any tendency for a high 



width . - . , . u , . , thickness . , 



z t- index to be associated with a high — -, -. — index, or 



length fe length 



vice versa. A glance at the correlation table for the ratios 



(Table VII) shews that there is some tendency for high values 



of each index to be associated, but that the correlation is far 



from precise. If we consider the full series of two hundred 



specimens, the coefficient of correlation of the two ratios has 



the value + 0481, while the equations of the regression are : 



(a) w =0-856 + 05156 I , 



(b) I =0-124 + 04485 



standard error + 0*0242 ; 



w 



l ' 



standard error +0*0226. 



It appears, however, that the value of the coefficient of 

 correlation as calculated from the data for the full series of 

 shells is unduly influenced by the presence amongst the series 

 of two individuals which have undergone during their growth 

 an injury which has led to their becoming quite noticeably 



