QUANTITATIVE STUDIES IN THE EVOLUTION 

 OF PECTEN. — III. 



COMPARISON OF PECTEN OPERCULARIS FROM THREE 

 LOCALITIES OF THE BRITISH ISLES. 



By C. B. Davenport. 



» Received August 29, 1903. 



Contents. 



Page 



I. Introduction 123 



II. Material 121 



III. Methods 126 



IV. Results 127 



1. Change of Proportions with Age 127 



2. Symmetry in Pecten 128 



3. Ray Frequency 132 



a. Correlation between Ray Frequency and the Dimensions of 



the Sliell 132 



b. Variation in the Number of Rays from the Three Localities . 132 



4. Individual and Specific Variability 135 



a. Geographical Variation of tiie Ears 135 



6. Variability of the Ears and their Correlation with the Antero- 

 posterior Diameter 136 



5. Color Variations and Abnormalities 137 



a. Interference with Development by Growth of Organisms on 



tlie Shell 137 



b. Abnormal Rays 138 



6. Definition of the Form Units 139 



a. Eddystone 140 



b. Firtli of Forth 140 



c. Irish Sea 140 



V. General Discussion 141 



1. Tlie Inconstancy of the Coefficient of Correlation in Allied Races . 141 



2. Geographical Variation in Relation to Evolution 142 



VI. Literature Cited 146 



I. Introduction. 



This investigation was undertaken witli the aim of finding out quanti- 

 tatively the way in which lots of individuals belonging to one species 

 but living in widely separated localities differ. The fact that such 



