8-t University of Washington Publications in Geology [Vol. 1 



extended; posterior dorsal margin slopes down from the beaks at an angle of 

 about 15 and gradually merges into the sharply arcuate posterior margin,, which 

 in turn merges into a very broadly rounded base; anterior margin sloping down 

 from the beaks very sharply at first and then much more gently to the anterior 

 end; anterior end truncated. A ridge or fold extends from the umbones to the 

 anterior basal margin. Posterior to this fold there is a faint concavity which 

 extends half-way down the surface and merges into the basal margin. 



Dimensions Altitude 15 mm.; longitude 20 mm.; thickness 6 mm. 



Occurrence At locality 367 (University of Washington Paleontological Col- 

 lection) situated on Gibson Creek, a few hundred feet up from the Northern Pacific 

 Railway crossing, in Section 2, Township 16 North, Range 5 West. 



Horizon Middle Oligocene; Turritella porterensis Zone. 



Gastropoda 



GENUS ACMAEA ESCHOCHOLTZ 

 ACMAEA OAKVILLENSIS 



Plate VII, Figure 18 



Description Shell small, thin and moderately low; apex situated about one- 

 third the length of the shell from the anterior end; anterior end with moderate 

 slope; outline of shell oval; surface sculptured with numerous radiating ribs with 

 wider interspaces. These are crossed by revolving lines of growth. 



Dimensions Altitude 4 mm.; diameter of base 13 mm by 9 mm. 



Occurrence At locality 161 (University of Washington Paleontological Col- 

 lection) situated about one mile west of Oakville on the Northern Pacific Railway. 



Horizon Lowermost Oligocene ; Barbatia merriami Zone. 



ACMAEA DICKERSONI n. sp. 

 Plate VII, Figure 15 



Description Shell small and high; apex situated about one-third the length 

 of the shell from the anterior end; base semi-oval in outline; anterior end steep. 

 Surface marked by numerous radiating ribs which are crossed by faint revolving 

 lines of growth. 



Dimensions Altitude 7 mm.; diameter of base 7 mm. by 10 mm. 



Occurrence At locality 161 (University of Washington Paleontological Col- 

 lection) situated about one mile west of Oakville on the Northern Pacific Railway. 



Horizon Lowermost Oligocene; Barbatia merriami Zone. 



