DAVENPORT. 



EVOLUTION OF PECTEN. 



139 



new shell is laid down unconformably upon old, one or more rays may be 

 incompletely or abnormally regenerated. 



6. Definition of the Form Units. 



It is clear from the foregoing sections that the pectens from the differ- 

 ent localities are sensibly different. Upon just such differences as these, 

 varieties have, in the past, been named and described at length, but 

 totally inadequately, by means of adjectives. I have no interest in 

 applying a name to the form units, but I think it is worth while to define 

 them quantitively, as in Table VIII. To be precise, this definition holds 

 only for the right valve of Pecten opercularis coming from the localities 

 which have been defined in detail and collected in October or November. 

 Indeed these definitions present only the condition, as near as half a 

 thousand random samples can give it, of the form units in the autumn of 

 1902. They afford standards of comparison with future years, which can- 

 not be said of qualitative definitions. 



TABLE VIIL 

 Proportions. 



Eddystone. Irish Sea. Firth of Forth. 



IMaximum dorso-ventral diameter t70 mm. 77 mm. *80 mm. 



Ratio a-p. to d-v. diam. when d-v. diam. = 67 mm. *L067 L061 tl.039 



liatio hinge length to a-p. diameter *0.507 0.483 tO.473 



Ratio anterior ear length to hinge length . . . *0.538 t0.520 0.533 



Ratio posterior ear length to hinge length . . . tO.462 *0.480 0.467 



Half globosity at length of 53 mm *0.151 *0.151 t0.132 



Average number of rays . . . 

 Standard deviation, rays . . . 

 CoelBcient of variability . . . 



Tlieoretical mode 



Skewness of distribution polygon 



Rats. 



Eddystone. 

 tl7.478 -i- .029 

 t 1.000 -J- .020 

 t 5.72 

 1 16.606 

 0.0802 





Irish Sea. 

 *18.101 -1- .029 



1.074 -J- .021 



5.93 

 *18.101 

 * 0.0870 



i-n% 



Firth of Forth. 

 17.673 -t- .027 



* 1.117 J- .019 



* 6.32 J- .11% 

 17.673 



t 0.0069 



Correlation Coefficients. 



Eddystone. Irish Sea. Firth of Forth. 



A-p. diam. and d-v. diam. tO.96997 ^ .00124 0.97596 -J- .0093 *0.9849o -j- .00063 

 D-v. diam. and partial 

 transverse diam. . . 0.8446 -J- .0063 t0.7840 ^ .0081 *0.8497 J- .0064 



Sum of symbols . . *4 -f 17 = 11 *5 + 12 = 7 *5 + t5 = 10 



Form unit witli the greatest number of extreme quantities : Eddystone. 



Form unit with the next greatest number of extreme quantities : Firth of Forth. 



Form unit with the least number of extreme quantities : Irish Sea. 



* Maximum. 



t Minimum. 



