18 University of Washington Publications in Geology [Vol. I 



posterior ventral line ; surface ornamented with medium, coarse, radiating ribs 

 which become obsolete on the middle region of the shell below the umbonal ridge. 

 In this respect the sculpture is like that of M. kelsoensis n. sp. 



This species differs from M. kelsoensis n. sp. with which it compares more 

 nearly in size and sculpture, in being less elongate but much broader and higher. 

 The general character of the sculpture of the two species M. kelsoensis and M. 

 columbianus is like that of the recent M. (Brachydontes) demissus (Dillwyn) of 

 the Atlantic coast, and which now, having been imported from the east, occurs 

 on the Pacific coast in San Francisco Bay. 



Dimensions. Length 35 mm. ; height 20 mm. ; width 20 mm. 



Occurrence. At locality 347 (University of Washington Palaeontological 

 Collection) on Coal Creek, Lewis County, back of Leybo place about one-half of 

 a mile below old Inman-Polson Camp, Section 27, Township 9 North, Range 3 



West. 



GENUS CRASS ATELLITES KRUGER 



CRASSATELPJTES STIUVWATERENSIS n. sp . 



Plate IX, Figures 1, 2 



Description. Shell of moderate size, thick and trigonal in outline; inequi- 

 lateral, beaks situated about one-third of the distance from the anterior end; 

 beaks small ; anterior margin sloping f rom the beaks at an angle of 30 for about 

 half the length of the anterior end and then rounding into the ventral margin; 

 ventral margin slightly curved ; posterior truncate ; extending vertically from the 

 ventral margin for about half the height of the shell and then sloping to the beaks 

 at an angle of 25; an umbonal slope extends from the beaks to the posterior 

 ventral margin; hinge heavy; large resilium scar; one large middle cardinal 

 with a thin anterior cardinal; margin of hinge plate grooved anteriorly; area of 

 lunule and escutcheon well developed ; internal margin smooth ; surface orna- 

 mented only with fine, concentric lines of growth. 



Dimensions. Length 43 mm.; height 32 mm.; thickness 18 mm. 



Occurrence. At locality 319 (University of Washington Palaeontological 

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

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



