8 University of Washington Publications in Geology [Vol. I 



GENUS ARC A (Linne) LAMARCK 



ARCA COI,UMBIANA n. sp. 



Plate VIII, Figure 1 



Description. Shell large and rectangular in outline ; nearly equilateral ; beaks 

 prominent, umbonal area broad ; hinge line straight ; cardinal area narrow ; dorsal 

 line sloping slightly from the beaks ; anterior end rounded, passing into the 

 ventral margin with a curve ; ventral margin only slightly curved, extending 

 abruptly into the posterior end ; posterior end straight ; posterior end is more 

 flaring and concave than the anterior end; there is 'a tendency for a slight 

 umbonal ridge to extend from the beaks to the posterior ventral margin ; shell 

 ornamented with 22 flat-topped, wide, very prominent radiating 1 ribs with inter- 

 spaces nearly equal to the width of the ribs. 



Dimensions. Length 40 mm. ; height 35 mm. ; thickness 30 mm. 



Occurrence. At locality 337 (University of Washington Palaeontological 

 Collection) on Coal Creek about one-half mile above old dam, Cowlitz County, 

 Section 11, Township 8 North, Range 3 West. 



SUBGENUS BARBATIA (Gray) ADAMS 

 ARCA (Barbatia) SUZZALLOI n. sp. 

 Plate VIII, Figure 6 



Description. Shell small, elongate in outline ; inequilateral, beaks situated 

 one-third the distance from the anterior end ; dorsal margin straight ; anterior 

 end nearly straight, sloping at about an angle of 15 with the dorsal margin, 

 passing into the ventral margin with a broad bow; ventral margin straight, 

 produced obliquely ; posterior margin broadly pointed at the ventral end, sloping 

 to the dorsal margin at about an angle of 40 ; a mid-umbonal sulcus extends to 

 the ventral margin, sloping obliquely from the beaks ; surface sculptured by con- 

 centric lines of growth crossed by numerous fine, delicate, radiating ribs which 

 bifurcate, forming two ribs of equal width on the lower region. The bifurca- 

 tion of the ribs occurs over the whole of the shell including the posterior as well 

 as the anterior region of concaveness. Under the microscope these ribs show 

 a beaded character, which are most numerous on the anterior, umbonal slope; 

 cardinal area very narrow. 



Named in honor of Dr. Henry Suzzallo, president of the University of 

 Washington. 



Dimensions. Length 20 mm. ; height 10 mm. ; width 8 mm. 



