STUDIES IN THE EVOLUTION OF PECTEN/ 

 IV. RAY VARIABILITY IN PECTEN VARIUS. 



BY 



C. B. DAVENPORT AND MARIAN E. HUBBARD. 



I. STATEMENT OF THE PROBLEM. 



Previous studies on the variability in the number of the rays of 

 Pectens have demonstrated that this number is, during develop- 

 ment, quite removed from any eflFect of external environment, 

 excepting only mutilation. The rays form, consequently, ex- 

 cellent material for the study of variability inherent in the germ- 

 plasm. Three species of Pecten have been previously quantita- 

 tively studied, all belonging to one group, sometimes known as the 

 subgenus Aequipecten (cf. Verrill, '99), characterized by large, 

 round, and nearly equal valves with fewer than 25 rays. It 

 seemed desirable to compare with these the variability of species 

 belonging to another group of the genus characterized by dorso- 

 ventrally elongated and skew valves and by more than 25 rays. 

 For this purpose Pecten varius seems admirably adapted. 



In comparing the variability of the rays of a species of the 

 varius type with that of species of the subgenus Aequipecten two 

 points of interest will be to learn whether the skewness remains 

 slight as in Aequipecten and, secondly, whether variability runs 

 parallel with the mean, increasing and diminishing with it. 



n. MATERIAL AND METHOD. 



The shells of Pecten varius used in this study were collected 

 between November 12, 1902, and February, 1903, from the 

 lagoon (Etang de Thau) between Cette and the mainland of 



'Titles of Nos. I to III of this series are given in the literature list as Davenport 

 1900«, 1903*' and 1903*^, respectively. 



