6io C. B. Dcnenport and Marian E. Hubbard. 



Pearson's Type I, with slight positive skewness. But the distri- 

 bution is so nearly normal that it may be taken as essentially so. 

 The following table gives a comparative view of the constants 

 for the different species of Pecten that have been quantitatively 

 studied. 



Table of Constants for Number of Rays in the Right (lower) Valve. 



n A a V a 



Pi.cten irradians: 



Cold Spring Harbor. .1046 17.353±.118 0.876±.013 5.049%dz.074 +0.023 

 Cutchogue, L. 1 281 16.534±.034 0.852±.024 5.15 %±. 15 +0.025 



Pecten gibbus: 



Dunedin (Tampa) , 



Fla 502 20.512 + .030 0.991+.021 4.83 %±. 10 +0.104 



Pecten ventricosus: 



San Diego, Cal 471 19.459 ±.087 0.885 + .019 4.55% ±.10 +0.015 



Pecten opercularis: 



Eddystone Liglit . . . . 536 17.478 + .029 1.000±.020 5.72 %±. 12 +.080 



IrisiiSea 614 18.101 + .029 1.074±.021 5.931%±.114 +.087 



Firth of Fortli 508 17.673 + .027 1.117 + .019 6.32 %±. 11 +.0069 



Perfen mriws, Cette . . 500 30.298 + .050 1.657 + .035 5.56% ±.11 +.067 



IV. CONCLUSIONS. 

 I. Skezuncss. 



Comparing the skewness of the frequency distribution o^ varius 

 with that of other species we find that, although the species marks 

 a departure from the more usual number of rays yet the skewness 

 of the distribution remains, like that of the other species, small. 

 The skewness in the Pecten ray distributions, so far as studied, 

 rises only exceptionally to o.i, although in other data this amount 

 of skewness is by no means uncommon. 



Now, large skewness is frequently, if not usually, associated 

 with recent or progressing evolutionary change. Thus in long 

 and in short-winged chinch bugs, which probably represent a 

 recent division of a monomorphic group, the skewness in the dis- 

 tribution of wing-length is —-43 and +.44, respectively; in the 

 short-horned race of rhinoceros beetles it is —.48. In the number 

 of lips of the newly arisen species of medusa, P. pentata, it is 

 — .55 (Mayer, 1901). In the shell-index of the newly introduced 



