1916] 



Packard: Mesozoic and Cenozoic Mactrinae 



273 



basis of considerable study, the writer would agree with Dall regard- 

 ing the constancy of these relationships and he would further state 

 that this distribution of the elements of the hinge offers the most 

 reliable known basis for the classification of this family. 



The greater constancy of the laterals as compared with the 

 laminae of the right valve, and the greater constancy of the form 

 of the left cardinal give the greater taxonomic value to the left 

 valve. 



A study of the cardinal tooth of the left valve shows that in 

 different species it occupies various positions on the hinge plate ; 

 from that at the dorsal margin of the plate, as in Mactra nasuta or 

 Spisula catilliformis, through a condition in which it is about cen- 

 trally located on the hinge plate, as in Mulinia densata, to that seen in 

 8. albaria or Mactra exoleta, in which the cardinal arises from the 

 ventral margin of the plate. The laterals in some cases arise con- 

 fluently with the edges of the hinge plate, as in 8. selbyensis or 

 Mxdinia densata, or they may be set some distance from the margin, 

 as in Mactra nasuta or 8. voyi. 



It has been shown above that the general shape of the shell, the 

 position of the umbones, and the character of the pallial sinus are 

 more inconstant than are the hinge characters, and therefore they 

 are only of secondary importance in defining a mactrine shell. 



In the systematic portion of this paper the species have been 

 grouped into subgenera and sections wherever it has been possible. 

 These lesser divisions are of use inasmuch as they show the relation- 

 ships that exist within a genus. The subgeneric groups, as defined 

 by Dall, are based upon the differences of the dental armature, due 

 consideration having been given to any other character of the shell 

 that appeared to have a taxonomic value. 



THE HISTORY OF THE MACTRINAE 



The geologic and geographic distribution of the West Coast 

 Mactrinae have a phylogenetic as well as an economic interest. The 

 oldest known species, Spisida ashburnerii (Gabb), has been reported 

 from the Horsetown Beds ; it has a long range, being found abund- 

 antly in the upper portion of the Chico, where it is associated with 

 8. gabbiana (F. M. Anderson) and S. chicoensis, new name. The 



