Studies in the Evolution of Pecten. 609 



it were, budded from the side of another. About 8 per cent of 

 the shells show marked breaks in the continuity of the surface, 

 due to the edge of the valves in the 15-20 mm. shell having met 

 at a less acute angle than earlier and later. Out of 500 shells 60 

 showed interpolated or double rays. 

 We found: 



8 extra rays i time. 



4 " " I " 



2 " " 14 times. 



I 44 



In the cases with two extra rays, one ray was long enough to 

 count in accordance with our rule in 3 cases; both rays were so 

 long in 6 cases; and neither was counted in 5 cases. Of the cases 

 with one extra ray, the ray was counted in 23 cases. 



Table of Ray-Frequencies, their Deviations and their Moments. 

 Classes f(v-Vo) f(v-Vo) i{v-Y,y f(v-Vo)=* f(v-Vo)^ 



149 1583 



vi=.298 ^2 =2.834 1^3=3.166 1^4=23.930 

 /i2 =2.7452 ,«3 =0.6853 m =21.6424 



The foregoing analysis of the distribution of frequencies by 

 moments shows us that the distribution apparently falls into 



