36 University of Washington Publications in Geology [Vol. 1 



ribs become almost obsolete. The incremental lines are sharp and wavy and in 

 the interspaces they curve downwards producing wavy lines which extend across 

 the entire shell. The ribbing is very minutely shown on the interior surface of 

 the shell. On the basal inner margin there are 24 flutings. Hinge set with 9 

 anterior and 9 posterior large and well developed V-shaped teeth. Area above 

 teeth with four reticulating ridges and grooves. Anterior and posterior muscle 

 scars are prominent and having a subrectangular outline. 



Dimensions Altitude 11 mm.; longitude 12 mm.; thickness 7 mm. 



Occurrence At locality 256 (University of Washington Palaeontological Col- 

 lection) in railway cuts on the O.-W. R. R. & N. Co. one fourth mile west and 

 north of Lincoln Creek Station in Section 27, Township 15 North, Range 3 West. 



Horizon Lowermost Oligocene which in this report is referred to as the 

 Porter Horizon or Molopophorous lincolnensis Zone. 



GENUS OSTRAEA LINNAEUS 



OSTRAEA LINCOLNENSIS n. Sp. 



Plate I, Figures 5 and 6 



Description Shell of moderate size, thin and sub-oval in outline; slightly in- 

 equilateral; lower valve convex with a somewhat uneven surface sculptured by con- 

 centric lines of growth. There is no evidence of radial ribbing. Fine crenula- 

 tions are present on the inner margins of both valves for a distance of one fourth 

 the length of the shell from the beaks on both the anterior and posterior dorsal 

 margins. 



Dimensions Altitude 65 mm. ; longitude 54 mm. ; thickness 20 mm. 



Occurrence At locality 256 (University of Washington Palaeontological Col- 

 lection) in railway cuts on the O.-W. R. R. & N. Co. one fourth mile northwest of 

 Lincoln Creek Station in Section 27, Township" 15 North, Range 3 West. 



Horizon Lowermost Oligocene; Molopophorous lincolnensis Zone. 



GENUS CRENELLA BROWN 

 CRENELLA PORTERENSIS n. sp. 

 Plate III, Figures 41 and 42 



Description Shell of medium size, thin, moderately inflated, elongate ovate 

 with distinct radial striations which diverge from a median line extending from 

 beak to basal margin. These striations consist of flat topped ribs with flat bot- 

 tomed interspaces of equal width. Beaks are small and terminal. Valves are al- 



