On the Variation of the Shell of Pecten irradians Lamarck from Long Island

The right or lower valve of P. irradians has on the average half a groove more than the upper, because the series of alternating ridges and grooves of the right valve has a prevailing tendency to end in grooves. Of three Long Island localities, the most nearly land-surrounded shows Pectens with the greatest number of rays. The right valve is less variable than the left, a result which agrees with the fact that the right valve of Pectens is generally more archaic than the left. The variation is nearly normal in both valves; more so in the left than in the right valve. The skewness is positive, showing a slight tendency towards an excessive production of the many-rayed individuals, or the selective annihilation of those with few rays. This positive skewness is paralleled by the fact that P. irradians seems to be developing towards a larger number of rays. The various abnormalities of Pecten are either explained as self-adjustments to accidents or as sports which represent typical conditions in allied species