Studies in the Evolution of Pecten. 6ii 



shore snail, Ltttorina littorea, of Newport, R. I., it is —.25. In 

 the ray-number of a selected 12-rayed race of Chrysanthemum it 

 is —.19, and soon. On the other hand only in some fossil Pecten 

 irradians of Virginia (rt=.22).do we find a skewness much exceed- 

 ing O.I. We may conclude, consequently, that, so far as the num- 

 ber of their rays is concerned, modern Pectens of the P. irradians 

 and P. varius types are not undergoing rapid evolution. 



2. Variability. 



The name varius implies an unusual variability in the species 

 we have studied. This variability is especially striking in the 

 color of the shell which varies from a straw color to a red and 

 almost to a black. We have seen, also, that there are frequent 

 abnormalities in the rays. Also, the standard deviation of the 

 ray-number, as the preceding table shows, is much higher than 

 in any of the P. irradians group, being about twice as great as in 

 the irradians of Long Island and 50 per cent greater than in the 

 opercular is from the Firth of Forth. The ray-number, then, 

 seems also to justify the name varius. 



But why is this species so variable although (as we may judge 

 from the small index of skewness) undergoing no rapid phylo- 

 genetic change? It appears to us that the high variability in the 

 number of rays depends in some way upon the great average 

 number of rays. Pecten varius, like its close allies, is characterized 

 by its great number of rays, a number hardly exceeded, so far as 

 I know, in the entire vast genus. The exceptional variability is, 

 indeed, closely proportional to the exceptional number of rays, 

 so that if we divide the index of variability by the average number 

 of rays we get for the resulting coefficient of variability of the form- 

 unit a number, 5.56 per cent, which is no longer extreme but lies 

 in the middle of the series of coefficients found in the Aequipecten 

 group, running from 4.55 per cent to 6.32 per cent. Thus, while 

 the index of variability is exceptional the coefficient of variability 

 is not at all so. 



The question arises. Why in a shell with many rays should the 

 index of variability of the number of rays be greater than in a shell 



