NO. 2116. ANATINACEA FROM WEST COAST OF AMERICA— BALL. 443 



by a rib, and there is no indication of a lithodesma. Length, 35; 

 height, 25; diameter, 12; vertical, from the posterior end, 16 mm. 

 Maximum length 40, height 30 mm. 



Distribution. — Arctic Ocean north of Bering Strait? Bering Sea 

 and Aleutian Islands, south and east to Sitka, Alaska. (Type 

 locality, Commander Islands.) Cat. No. 221555, U.S.N.M. 



THRACIA CHALLISIANA, new species. 



Shell large, chalky, subquadrate,* almost equivalve, the posterior 

 end longer, surface covered with a coarse, almost imbricating granu- 

 lation, without radial elevations or defined dorsal areas. Both ends 

 are rounded, there is no distinct posterior tnmcation, though that 

 end is blunter than the anterior; resilium and ligament marginal, 

 inconspicuous, with no indication of a lithodesma; pallial sinus 

 wide, shallow, reaching only halfway from the posterior end to the 

 vertical of the beaks. Length, 45; height, 33; diameter, 20; vertical 

 from the beaks 25 mm. in front of the posterior end. 



Distribution. — ^Type locality, San Juan Island, Gulf of Georgia. 

 Cat. No. 272096, U.S.N.M. Also at Forrester Island, Alaska. 



This species, which is allied to the fossil T. condoni Dall (1909), 

 is named in honor of Miss Bertha M. Challis of the State Museum, 

 Seattle, Washington, who has exhibited much interest in the moUusca 

 of the region. 



THRACIA DIEGENSIS, new species. 



Shell very thin and smaU, whitish or brownish, usually with a 

 deposit of rusty orange color about the margin; valves moderately 

 subequally convex, the anterior end longer, the posterior end attenu- 

 ated and squarely truncate; posterior dorsal area defined by a 

 raised thread on each valve; surface smooth, except for incremental 

 irregularities; beaks rather prominent; no perceptible granulations; 

 hinge linear and very feeble, the resihfer not projecting; in the 

 cavity of the beak a short line of brown matter resembling cement 

 extends downward, which indicates the attachment of a widely V- 

 shaped lithodesma which is usually lost; palHal sinus deep, reaching 

 beyond the vertical of the beaks; base of valves nearly straight. 

 Length, 9; height, 7; diameter, 4 mm.; vertical of the beaks 3 mm. 

 in front of the posterior end. 



Distribution. — ^Type locahty, San Diego Bay, in 1 to 5 fathoms 

 sandy mud, very abundant. Cat. No. 73604, U.S.N.M. 



This little species resembles T. fdbula Philippi, from Sicily. It 

 is one of the few species which seem to flourish on the muddy bottom 

 of the bay. 



THRACIA COLPOICA, new species. 



This is a species belonging to the same type as T. diegensis but 

 larger, more rounded, with a more irregular surface; the posterior 



