Studies in the Evolution of Pecten. 615 



error as a percentage of that length. Nevertheless, the size of the 

 absolute error is of great moment in such a survey. For an error 

 in levels of one meter at the end of a 100 kilometer course is 

 practically much more important than an error of a centimeter at 

 the end of a single kilometer course. Exactly, then, as absolute 

 and proportional errors have both their importance in engineering, 

 so neither the index of variability nor the coefficient can be 

 neglected in biology. Each has its peculiar significance. 



V. SUMMARY. 



Pecten varius of Cette has a large number of rays as compared 

 with representatives of other subgenera of Pecten. The fre- 

 quency distribution has a small skewness as in other subgenera 

 indicating that the species is probably not evolving rapidly. The 

 variability of the ray frequency is large when measured by the 

 index, but the same as in other species with fewer average rays 

 when measured by the coefficient of variability. Both index and 

 coefficient give important insight, each of its own kind, as to the 

 degree and nature of the variability. 



YI. LITERATURE CITED. 



Brewster, E. T., 1897. — A Measure of Variability and the Relation of Individual 

 Variation to Specific Differences. Proc. Amer. Acad. Arts and Sci., 

 XXXII, 268-280. 



Davenport, C. B., 1900^. — On the Variation of the Shell of Pecten irradians La- 

 marck from Long Island. Amer. Nat., XXXIV, 863-877. 

 1903''. — A Comparison of the Variabilitj'- of Some Pectens from the East 

 and the West Coasts of the United States. Mark Anniversary- 

 Volume, pp. 121-136, Plate IX. New York: Henry Holt & Co. 

 1903'^. — Quantitative Studies in the Evolution of Pecten. III. Com- 

 parison of Pecten opercularis from three localities of the British 

 Isles. Proc. Amer. Acad. Arts and Sci., XXXIX, 123-159, Nov. 



DuNCKER, G., 1900*'. — On Variation of the Rostrum in PaLTmonetes vulgaris 

 Herbst. Amer. Nat., XXXIV, 621-633, Aug. 



Mayer, A. G., 1901. — The Variations of a ncAAiy-arisen Species of Medusa. Science 

 Bulletin, Museum Brooklyn Institute of Arts and Sciences, I, 1-22, 

 2 pis., April. 



Pearson, K., 1895. — Contributions to the Mathematical Theory of Evolution. II. 

 Skew Variation in Homogeneous Material. Phil. Trans. Roy. Soc. 

 London, CLXXXV, A, 71-110., pis. 1-5. 



