1916] Packard: Mesozoic and Cenozoic Mactrinae 263 



It is the purpose of this paper to apply the taxonomic system 

 proposed by Dr. W. H. Dall to the West Coast mactrine species, to 

 record the known variation found within each species of this sub- 

 family, to discuss the geologic and geographic ranges of the different 

 species, to determine the value of each species to the stratigraphic 

 geologist, and incidentally to describe several forms that are appar- 

 ently new to science. 



The term Pacific Coast region as vised above is intended to include 

 only the territory from Mexico northward to the coast of Alaska, 

 and extending eastward to the Sierra Nevada and the Cascade 

 mountains. 



A large number of fossil and living Mactrinae belonging to the 

 University of California and the University of Washington have 

 been available for study. These collections include several types of 

 species and varieties. 



The writer wishes to express his thanks to Dr. John C. Merriam 

 for the general supervision of this paper; and to Dr. Charles E. 

 Weaver of the University of Washington, who loaned several type 

 specimens and gave much valuable assistance during the early stages 

 of this study. 



HISTORICAL SKETCH 



During the period in which the West Coast species were described 

 the basis of classification of the Pelecypoda changed considerably. 

 Gray 1 in his study of the Mactridae, in 1837, differed from his 

 predecessors in that he based his classification largely upon the posi- 

 tion of the ligament, whereas the earlier naturalists had considered 

 the general shape, the pallial line, or some minor anatomical character 

 as having the largest taxonomic value. Gray's system, modified by 

 such workers as Reeve 2 , Conrad, and Gabb, was in general use until 

 Dall 3 in 1889 demonstrated the value of the pelecypod hinge as a 

 basis of classification. Subsequent investigators have accepted Dall's 

 conclusions and have successfully applied his methods in the separa- 

 tion of the subordinate groups of the Pelecypoda. 



1 See Deshayes, G. P., Traite elementaire de conchyliologie, torn. 1, pt. 2, 

 p. 256, et seq., 1843-1850. 



2 Reeve, L. A., Conchologia Iconica, "Monograph of the Genus Mactra," 

 vol. 8, 21 pis., April 1, 1854. 



3 Dall, W. H., On the hinge of the Pelecypoda and its development, etc., 

 Amer. Jour. Sci. (3), vol. 38, pp. 445-462, 1889. 



