482 THE AMERICAN NATURALIST. [Vol. XXXVII 



species, but the knowledge of their variations. Moreover, the 

 beginning of all investigation into the causes of those variations 

 is the knowledge of the direction which they take, whether they 

 are promiscuous or whether they bear some definite relation to 

 each other or to the environment." 



The following paper is a contribution to the study of variation 

 in the sculpture of the pelecypod Mollusca, as shown in the 

 genus Cardium. 



II. Material. 



The material upon which this paper is based consists of 

 three species of Cardium ; C. robiistum, C. isocardia, and 

 C. muricatum. They were collected by Dr. J. W. Velie and 

 W. W. Calkins on the west coast of Florida, near Tampa. 

 Several hundred specimens of each species have been examined, 

 thus affording enough material for a wide range of variation. 

 To this material is added some data gathered by Dr. \Y. M. 

 Dall and published in his « Contributions to the Tertiary Fauna 

 of Florida." 



The ribs on each valve- were carefully counted and in order to 

 remove any possibility of error they were recounted several 

 times at intervals of two or three days. The size (length) of 

 each shell was determined by a pair of calipers spread from the 

 umbones to the ventral margin. These measurements are all in 



In the curves plotted, the groups or classes having the same 

 number of ribs are indicated on the horizontal line, and the 

 number of specimens in these groups, the frequencies, are noted 

 on the vertical line. 



In the tables the number of ribs is indicated as a numerator, 



