18 University of Washington Publications in Geology [Vol. 1 



very slightly concave and sculptured by five narrow, rounded revolving ribs be- 

 tween each of which there are flat botttomed interspaces of double their width ; 

 on the angulated portion of each whorl there are three cord like revolving ribs 

 with interspaces of equal width ; immediately below the angle there are ten ribs 

 with flat surfaced interspaces three to four times wider than the ribs ; near the an- 

 terior end of the whorl the ribs become fainter and closer together. The revolving 

 ribs are crossed by 13 longitudinal ribs which produce prominent nodes on the 

 angulated portion of the whorls but are only faintly noticeable above and below 

 the angles. Aperture sub-oval; outer lip arcuate and thin; inner lip smooth; col- 

 umella short and curved with two moderately developed plications and a small 

 anterior sulcus. 



Dimensions Altitude of shell 9 mm. ; altitude of spire 4 mm. ; maximum 

 diameter of shell 7 mm.; angle of spire 55. 



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 STREPSIDURA SWAINS 



STREPSIDURA WASHINGTONENSIS n. sp. 



Plate V, Figures 81, 82 and 83 



Description Shell of medium size with six and one-half whorls ; spire mod- 

 erately elongate and about four-fifths as long as the body whorl; body whorl 

 sculptured with two well pronounced revolving ridges and a third one less well 

 defined and situated anterior to the other two. Suture distinct; upper portion of 

 body whorl tabulated and slightly concave; surface between the two revolving 

 ridges very slightly concave; revolving ribs crossed by twelve well defined longitud- 

 inal ribs which begin upon the upper surface of whorl a short distance from the 

 suture and extend over the entire surface until within a short distance of the end 

 of the canal; sharp angular nodes are developed at the points where they cross the 

 three revolving ridges; upper surface of whorl sculptured with 15 revolving 

 threads between each of which there is a flat surfaced interspace of equal width; 

 there are seven similar revolving threads on the middle of the whorl and between 

 each a very broad interspace within which is set a minute revolving thread; ante- 

 rior portion contains 15 revolving threads with interspaces three times as wide. 

 All the whorls of spire distinctly sculptured; aperture sub-elliptical; outer lip rel- 

 atively thin and smooth; inner lip heavily callused, posterior end of aperture broad; 

 anterior end narrow and extended into an elongated and reflected canal; a well 

 defined fascicle is present; plications are absent on inner lip. 



This species is close to Strepsidura oregonensis Dall. It differs in that 



