1916] Packard: Mesozoic and Cenozoic Mactrinae 265 



Wagner Free Institute of Science 8 . These last two papers contain 

 excellent definitions of the genera, subgenera and sections into which 

 the family had been divided previously. The many cuts of the last 

 paper showing the important taxonomic characters of the different 

 groups add much to the usefulness of the paper. Between the years 

 1894 and 1909, Dall described the following: Mactra hemphillii, 

 Mulinia oregonensis, Spisula alashana, 8. callistaeformis and 8. 

 precursor. 



In the meantime P. M. Anderson described Mactra gabbiana, 

 and Dr. Ralph Arnold added the following : Mactra coaling ensis, 

 M. montereyana, M. stantoni, Mulinia densata var. minor, Spisula 

 catilliformis var. alcatrazensis and S. sisquocensis. 



Dr. C. E. Weaver in 1912 reported the following from the 

 Miocene of Washington: Pseudocardium landesi, P. gabbi var. altum, 

 P. gabbi var. elongatum, P. gabbi var. robustum and P. gabbi var. 

 unduliferum. 



BASIS OP CLASSIFICATION 



Certain shell characters that are now considered of taxonomic 

 value were omitted in the early descriptions of the West Coast Mac- 

 trinae, because systematists did not regard them as of importance. 

 The shell characters such as general shape, or position of the umbones, 

 which were considered by these earlier writers are now known to be 

 too variable to be of much value to the systematise A number of Con- 

 rad's mactrine species were based upon specimens too immature for 

 specific determination. This was recognized by Carpenter, who in 

 speaking of Conrad 's early descriptions, says : 1 ' Conrad 's types being 

 lost, and his species imperfectly described from very young specimens, 

 a difficulty attends their identification." 9 



In reviewing the mactrine species it has been found necessary to 

 apply a standard to each described form. A study of the specific 

 variations within this subfamily has revealed the fact that the most 

 constant shell characters within a species are those dependent upon 

 the dentition. Dall was the first to recognize the value of the hinge 

 of the Maetridae as a satisfactory basis of classification. He provided 

 a special nomenclature for the hinge dentition of these forms in his 



s Dall, W. H., Maetridae, Transactions Wagner Inst. Sci., vol. 3, pt. 4, 

 pp. 862-891, 1898. 



s Carpenter, P. P., Brit. Assn. Rept. 1863, p. 99. 



