CLASS PELECYPODA 
85 
Type in U. S. N. M., No. 272096. Type locality, San Juan Island, 
Washington. 
Range. Forrester Island, Alaska, to the Gulf of Georgia. 
Thracia beringi Dali, 1915. 
Plate 43, fig. 4. 
Proc. U. S . N. M. y 49:442. 
Shell of moderate size, white, with a dehiscent, yellowish periostracum 
(usually lost), and looking much like a Maconia sabulosa. Valves nearly 
equal, nearly equilateral, with low beaks and a conspicuous, external liga¬ 
ment enfolding the resilium. Surface with incremental irregularities and 
in the periostracum numerous small wrinkles, but without perceptible 
granulation. Pallial sinus large, rounded, not reaching the vertical from 
the beaks. Valves rounded at both ends, the posterior dorsal area incon¬ 
spicuous, not bounded by a rib, and there is no indication of a lithodesma. 
Length, 35; height, 25; diameter, 12; vertical from the posterior end, 
16 mm. (Dali.) 
Type in U. S. N. M., No. 221555. Type locality, Commander Islands. 
Range. Bering Sea, Aleutian Islands and southward to Sitka, Alaska. 
Thracia diegensis Dali, 1915. 
Proc. U. S. N. M., 49 :443. 
Shell very thin and small, whitish or brownish, usually with a deposit 
of rusty orange color about the margin; valves moderately, sub-equally 
convex, the anterior end longer, the posterior end attenuated and squarely 
truncate; posterior dorsal area defined by a raised thread on each valve; 
surface smooth, except for incremental irregularities; beaks prominent, no 
perceptible granulations; hinge linear and very feeble, the resilifer not 
projecting; in the cavity of the beak a short line of brown matter resem¬ 
bling cement extends downward, which indicates the attachment of a 
widely V-shaped lithodesma which is usually lost; pallial sinus deep, 
reaching beyond the vertical of the beaks; base of valves nearly straight. 
Length, 9; height, 7; diameter, 4 mm. (Dali.) 
Type in U. S. N. M., No. 73604. Type locality, San Diego Bay, 
California. 
Range. Known only from the type locality. 
Genus CYATHODONTA Conrad, 1849. 
Shell like Anatina in form; hinge with a broad, not very projecting, 
fossette which is carinated near the margin; muscular impressions rounded, 
indistinct; pallial impression with a large, rounded sinus. 
Type. Cyathodonta undulata Conrad. 
Distribution. China, California, Lower California, Honduras. 
