Clark: The San Lorenzo Series of Middle California 139 



Empire beds (Upper Pliocene) of Coos Bay, Oregon, which he refers 

 to as a young form of P. acutilineatus. This does not agree with 

 Conrad's figure nor with the species determined by the writer as 

 P. acutilineatus from Oregon and Washington. The specimen figured 

 by Dall has a different apical angle from Conrad's species and it lacks 

 the marked depressed anterior and posterior areas. The specimens 

 of Phacoidcs obtained from the Astoria series of Mount Diablo are 

 too poorly preserved for us to be certain of the specific determination- 



Family DlPLODONTIDAE 

 Genus P-IPLODONTA Brown 

 DIPLODONTA STEPHENSONI, n. sp. 

 Plate 12, figure 6 

 Type specimen 11171, Coll. Invert. Palae. Univ. Calif., loc. 1131 



Shell subcircular, medium in size, only moderately inflated; beaks nearly 

 central, rather inconspicuous; height and length about equal. Posterior dorsal 

 edge straight ; about equal to anterior dorsal edge which is straight to slightly 

 convex. Posterior end broadly subtruncate ; anterior end very broadly rounded. 

 Ventral edge gently and regularly arcuate. Surface smooth except for medium 

 fine incremental lines of growth. Nymph plates heavy, fairly high with no 

 well-defined resilifer pit. In the right valve, the posterior cardinal and in the 

 left valve the anterior cardinal is so deeply bifid that each has the appearance of 

 being two distinct teeth instead of one. 



Dimensions. — Type specimen: height, 8 mm.; length, 8 mm.; diameter of 

 one valve, about 2.5 mm. 



Occurrence. — University of California locality 1131. 



Named in honor of L. W. Stephenson, palaeontologist in the United States 

 Geological Survey. 



Diplodonta stephensoni somewhat resembles in outline I). serri- 

 cata Reeve, 110 a Recent West Coast species; the two are only slightly 

 different in outline ; the beaks of the latter are possibly more conspicu- 

 ous and the shell slightly more inflated. The chief difference between 

 the two is the hinge plate. D. serricata has a well-defined resilifer 

 pit while D. stephensoni has not; on the latter species the nymph 

 plates are heavier and better defined ; also the posterior tooth of the 

 right valve and the anterior of the left valve of the latter species are 

 more deeply bifid than on the former. 



116 For a good description and figures of D. serricata see Arnold, Mem. Calif. 

 Acad. Sci., vol. 3, p. 134, pi. 18, figs. 5, 5a, 1903. 



