290 University of California Publications in Geology [ v °l. 9 



This species is extremely variable in general shape. It is typi- 

 cally more quadrate in outline than the associated mulinoid species, 

 and it lacks the prominent beaks of these species, besides differing 

 in details of the hinge. The type of this species was obtained from 

 the indefinite locality of San Pablo Bay, Contra Costa County, 

 California. 



Occurrence. — Miocene : Lower San Pablo group, Mount Diablo 

 region, Contra Costa County, California. 



SPISULA ALB ARIA (Conrad) 

 Plate 24, figure 1; plate 25, figures 3 to 8 



Mactra albaria Conrad, Am. Jour. Sci., 2d. ser., vol. 5, p. 432, fig. 4, 1848; 

 Dall, U. S. Geol. Surv. Prof. Paper 59, p. 150, fig. 4, 1909 (reprint); Arnold, 

 U. S. Geol. Surv. Bull 396, p. 30, pi. 19, fig. 4, 1909. 



Mactra diegoana Conrad, House Doc. no. 129, p. 14, 1855; Pacific R. R. 

 Repts., vol. 5, p. 325, pi. 5, fig. 45, 1855. 



Standella planulata, Gabb, Pal. Cal., vol. 2, p. 91, 1869; not of Conrad 1837. 



Spisula albaria (Conrad), Dall, U. S. Geol. Surv. Prof. Paper 59, p. 130, 

 pi. 10, fig. 1, 1909. 



f Spisula precursor Dall, U. S. Geol. Surv. Prof. Paper 59, p. 131, pi. 14, 

 fig. 10, 1909. 



Conrad described this species as follows : 



Triangular, ventricose; beaks medial; umbonal slope angulated; anterior 

 and posterior margins nearly equally oblique; posterior extremity trun- 

 cated, direct; basal margins regularly curved. 



The hinge of a specimen from the Merced group, near Capitola, 

 Santa Cruz County, CaHfornia, is best known and will serve as the 

 basis for the description. 



Hinge plate wide, chondophcre oblique, rather deep and par- 

 tially roofed by an accessory ; spur distinct in most specimens ; car- 

 dinals large, heavy, arms widely diverging ; dorsal edge of the left 

 cardinal is fused to the margin of the shell; ventral sinus some- 

 what excavated; right cardinal prominent, anterior arm short, pos- 

 terior arm heavy, extending in a straight line from near the beak 

 to the ventral margin of the plate ; anterior lateral short, top 

 evenly rounded, smooth, distant from and in alignment with the 

 anterior arm of the cardinal ; posterior lateral long, slightly higher 

 than the margin of the shell arising from the ventral edge of the 

 hinge plate ; laminae long, the ventral ones also arising from the 

 edges of the plate; the dorsal ones are small, arising from the 



