22 University of Washington Publications in Geology [Vol. I 



size ; dorsal margin sloping from the beaks, posterior end passing from the dorsal 

 line at an angle of 30 forming a broadly pointed margin, ventral margin regu- 

 larly curving from the posterior margin and rounding into the anterior end; 

 anterior slightly produced, concave beneath the beaks ; surface sculptured with 

 prominent, wide, concentric ribs which cover the entire surface of the shell; the 

 interspaces very narrow, about one-fourth of the width of the ribs. 



This species differs from Meretrix uvasana Gabb, Meretrix tejonensis Dick- 

 erson and Meretrix uvasana Conrad (Dickerson) in being 1 more elongate in the 

 outline of the shell, and the ribs in M. williamsoni are very wide with narrow 

 interspaces while on the species mentioned the ribs are narrow with wide inter- 

 spaces. M. williamsoni differs from Macrocallista conradian Gabb in being 

 less narrowly pointed posteriorly and less elongate. 



Named in honor of the late Mrs. Martha Burton Williamson of Los Angeles, 

 California, who has been an ardent contributor to the conchology of the Pacific 

 Coast. 



Dimensions. Length 39 mm. ; height 28 mm. ; thickness 10 mm. 



Occurrence. At locality 319 (University of Washington Palaeontological 

 Collection), MicClarety ranch on south bank of Stillwater Creek, one mile 

 west of Vader, Lewis County, Section 30, Township 1 1 North, Range 2 West. 



GENUS PSAMMOBIA LAMARCK 



PSAMMOBIA COUJMBIANA n. Sp. 



Plate X, Figure 18 i 



Description. Shell large and oblong ; inequilateral, the anterior end shorter ; 

 posterior end higher than the anterior; anterior and posterior dorsal lines 

 straight and oblique, the anterior sloping at a greater angle; posterior and dorsal 

 ends rounded below ; ventral margin straight ; surface of shell with coarse, con- 

 centric lines of growth. 



Dimensions. Length 75 mm. ; height 36 mm. ; thickness 19 mm. 



Occurrence. At locality 342 (University of Washington Palaeontological 

 Collection) on Coal Creek, Cowlitz County, about one-fourth of a mile above 

 Inman-Polson shops, Section 2, Township 8 North, Range 3 West. 



PSAMMOBIA COWUTZENSIS n. sp. 

 Plate IX, Figure 18 



Description. Shell large and oblong in shape ; inequilateral ; anterior end 

 shorter, convex ; anterior dorsal margin sloping ; shell rounded at both ends ; 



