16 University of Washington Publications in Geology [Vol. I 



posterior end; umbones strongly curved, forming a sharp ridge on the dorsal 

 margin; umbonal slope extending from the beaks to the posterior end; in 

 younger specimens the dorsal margin of this ridge is more rounded; sculpture 

 delicate, consisting of flat, radiating ribs which are larger over the dorsal and 

 posterior regions continuing to the lower surface of the umbonal slope ; the ribs 

 on the middle portion of the shell are very much finer and more delicate, increas- 

 ing in size on the short anterior end; hinge margin crenulated, distinct on the 

 anterior portion, very fine beneath the beaks and becoming obscure on the 

 posterior region; shell with a beautiful nacreous luster. 



Dimensions. Length 21 mm. ; length of umbonal slope 20 mm. ; thickness 

 7 mm. 



Occurrence. At locality 324 (University of Washington Palaeontological 

 Collection) in the west bank of the Olequah Creek, about one-eighth of a mile 

 north of Vader, Lewis County, Section 29, Township 11 North, Range 2 West. 



MoDioiyUS (Brachydontes) COWUTZENSIS n. sp. 

 Plate IX, Figure 19 



Description. Shell medium in size ; subovate in outline ; dorsal line straight, 

 curving regularly into the posterior end which extends almost vertically down- 

 ward ; posterior end broad and inflated ; anterior end short ; beaks low ; umbonal 

 slope prominent and arching downward, convex above with a prominent concavity 

 beneath; surface ornamented with radiating ribs which over the posterior and 

 umbonal regions are large and flat with narrow interspaces ; on the middle por- 

 tion of the shell the ribs are very fine and delicate; on the anterior end of the 

 shell, there are five or six radiating ribs which are enlarged with wide spaces 

 between, the interspaces equal to the width of the ribs ; cardinal margin denticu- 

 late, the series of teeth are larger on the anterior margin, diminishing in size 

 beneath the beaks and becoming obscure on the posterior end ; shell very thin and 

 fragile. 



The sculpture of this species is like that of M. olequahensis n. sp. which has 

 the tripartite differentiation of the radiating ribs. It differs from M. olequah- 

 ensis in the distinct curving of the umbonal ridge and the almost straight vertical 

 slope of the posterior end. The area above the umbonal slope in M. cowlitzensis 

 is convex and slightly concave near the margin of the shell, the greatest con- 

 cavity being beneath the ridge while in M. olequahensis the area above -is concave 

 and only slightly concave beneath. 



Dimensions. Length 1 5 mm. ; thickness 7 mm. 



