1922] Fauna from the Eocene of Washington 25 



tral end; posterior end sloping from the dorsal line at an angle of 30, narrowly 

 truncate at the base ; a sharp ridge extends from beaks to the posterior ventral 

 margin ; valves very convex ; left valve tends to be more pointed posteriorly and 

 the ventral line less straight ; surface ornamented with fine, distinct concentric 

 ribs with interspaces about half the width of the ribs; concentric ribs terminate at 

 the umbonal ridge, the area between the ridge and the anterior margin smooth 

 except for lines of growth. Crossing the concentric ribs there are commonly 

 threadlike, radiating lines on the lower portion of the shell, which extend from a 

 mid-line to the umbonal ridge. These lines are very fine and often microscopic 

 with very wide interspaces between, where they cross the concentric ribs they 

 give the surface a cancellated appearance. 



This form differs from Corbula hornii Gabb in being less equilateral, in the 

 anterior end being less produced and the posterior end more pointed. 



Named in honor of Dr. Roy E. Dickerson of the California Academy of 

 Sciences, San Francisco, California. 



Dimensions. Length 12 mm. ; height 8 mm. ; thickness 6 mm. 



Occurrence. At locality 329 (University of Washington Palaeontological 

 Collection) west bank of the Cowlitz River near Vader, Lewis County, Section 

 28, Township 11 North, Range 2 West. 



CORBUI,A STJI^WATDRENSIS n. Sp. 



Plate VIII, Figure 14 



Description. Shell large, sub-ovate; rostrate; beaks large and prominent; 

 valves very convex ; dorsal margin slightly convex, anterior end rounded ; ven- 

 tral margin rounded anteriorly, sloping slightly upward in the region of the 

 rostrated posterior end; extreme margin of the posterior end broken; dorsal 

 margin concave, sloping to the beaks at an angle of 40 ; shell thin, smooth ex- 

 cept for the fine lines of growth. ^ 



Dimensions. Length 22 mm. ; height 16 mm. ; thickness 13 mm. 



Occurrence. At locality 319 (University of Washington Palaeontological 

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

 west of Vader, Lewis County, Section 30, Township 11 North, Range 2 West. 



