270 University of California Publications in Geology [ VoL - 9 



The charts give no definite clue to the relative variations 

 within genera, for each genus includes species of a great range of 

 variability. This inconstancy of form-variation is not confined to 

 the subfamily, since it is also evident in the lutrarine species 

 Schizothaerus nuttallii (fig. le). 



If each of the specimens within the groups charted above had 

 not possessed characters that were constant for the given species, 

 it might be thought that measurements had been taken upon speci- 

 mens representing more than a single species. Not only did each 

 suite of shells have constant characters, but in every instance there 

 were gradational forms connecting the extremes within the group. 



If it be granted that the conclusions, based upon the measure- 

 ments and other less precise methods given above, are legitimate, 

 it is manifest that for some species, at least, the general outline of 

 the shell is not a character of any considerable taxonomic value. 

 Although it serves admirably in some instances, it fails in many 

 others and therefore it cannot be employed as a specific criterion 

 that is equally applicable to all of the members of this subfamily. 



The prominence of the beak and its position with reference to 

 the anterior extremity, have often been used as important specific 

 characters in the definitions of mactrine species. The relative 

 amount of shifting of the umbones within the limits of a single 

 species is shown in figure lg. In this graph the abscissas represent 

 the distance from the umbones to the anterior extremity, measured 

 along a line parallel to that of the greatest anterioposterior 

 diameter, and the ordinates represent the greatest length of the shell. 

 The same suite of specimens of Spisula selbyensis used in the previ- 

 ous instance was made the basis for these measurements. The 

 scarcity of parallel lines in this chart indicates the marked amount 

 of variation in the position of the beaks. Upon comparison with the 

 graph of length-height ratios of the same species (fig. 1/) it is found 

 that these ratios are more nearly constant than are those based 

 upon the relative position of the umbones. Similar conditions 

 occur in Mulinia densata, M. pabloensis, Spisula albaria and 8. 

 abscissa. 



The shape and the prominence of the umbones are noteworthy 

 variants within the limits of certain species such as Mulinia densata. 

 In the more ventrieose Psetidocardium forms the beaks are very 

 prominent, in marked contrast to the small ones of the typical form. 



