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University of California Publications in Geology [Vol. 9 



limits of certain species ; while others appear quite constant when 

 the same character is considered. 



This form-variation is rendered comprehensible by graphic 

 methods. In figure 1 the abscissas represent the height and the 

 ordinates the length of the shell. A line connecting the two points 

 which correspond to these dimensions represents the ratio of the 

 length to that of the height of the shell. These measurements, 

 based upon a number of specimens, when plotted in this manner 

 show a series of lines in which deviation from parallelism indicates 

 the inconstancy of these ratios for the specimens under consideration. 



The graphs show that Spisula selbyensis (fig. 1/) is the least 

 constant, and that Mactra calif or nica (fig. lc) is the most constant 

 of the mactrine species represented. The graphs of 8. ashburnerii 

 (figs. Id, lc) and 8. merriami, show but little variation, suggesting 

 that they are readily separable upon the basis of their general shape. 

 Mulinia densata and 6'. selbyensis are among the more variable forms 

 shown. 



The species represented that are shown to be the more variable, 

 are difficult of determination. Mulinia densata and Spisula selbyensis 

 are often confused with such forms as M. pabloensis and S. albaria, 

 or even with other less closely related species. The variations in the 

 general shape of 8. catilliformis, especially in the case of juvenile 

 specimens, include forms closely resembling Schizothaerus nuttallii. 



Figure la shows the variations of Spisula catilliformis through 

 several geologic periods. This graph is based upon specimens from 

 different beds ranging in age from Lower San Pablo to and including 

 the Eecent. Such studies suggest that as time advanced this species 

 became more constant as regards general shape. Specimens from 

 the Lower San Pablo show shape-variations greater than have been 

 observed in the living species. This, together with evidence shown 

 by the graphs of Mulinia densata (fig. lb) and Spisula selbyensis 

 (fig. 1/), and still further augmented by a consideration of a number 

 of species not here plotted, suggests a remarkable inherent variability 

 of the Miocene species in contrast to those of earlier or later periods. 

 No adequate explanation of this has been found, unless it be that 

 this indicates a period of specific unstability due perhaps to adapta- 

 tions to the shifting local environments of the Miocene and to the 

 aggressiveness or capacity for variation often characterizing the 

 earlier stages in the life-history of a genus or other group. 



