204 AXEL A. OLSSON 



Shell small or of medium size, obliquely subcircular to orbicular, thin, 

 depressed to slightly convex, white, the periostracum is thin and of a pale 

 horn color to nearly white. The umbones are high, narrow, and project well 

 above the hinge line, the beaks small and prosogyrous. The anterior slope is 

 depressed, the margin itself straight or appears as if widely truncated, the 

 posterior side longer, rounded at the end and a part of the circular curve 

 which forms the ventral margin. Surface with an even sculpture of coarse, 

 raised, concentric threads recalling that of Dosinia dunkeri spaced between 

 slightly wider intervals. Ligament area excavated into the hinge plate, 

 short. 



Length 19 mm., height 18.2 mm., semidiameter 4.8 mm. Tumbez. 



This species differs from D. tellinoides, with which it may occur, by 

 its more oblique form and especially by its high, narrow, prominent um- 

 bones which project conspicuously above the dorsal margin. Its surface 

 sculpture is neat and regular, formed by evenly distributed concentric 

 threads which were likened to the surface pattern of Dosinia dunkeri by 

 Dall. 



Range — Gulf of California to northern Peru. Ecuador: Santa Elena. 

 Peru: Tumbez. 



Genus PHLYCTIDERlffA Dall, 1889 

 Type species by original designation, Diplodonta sevuaspera Philippi. 

 Recent, West Indies, and the Caribbean, 



Shell subcircular, convex or plump, semisolid. Hinge stout, the left 

 valve with a large, hook-shaped or projecting, bifid, anterior cardinal tooth, 

 the posterior cardinal simple, smaller, obliquly attached to the end of the 

 nymph; the right valve has the posterior cardinal bifid, bordered in front 

 by a deep socket. Ligament external but lies deeply immersed in an exca- 

 vated furrow well below the valve margin, the nymph itself small, narrow, 

 linear in shape. External surface coarsely sculptured with concentrics, often 

 with large nodes or V-shaped pustules over most of the disk but lacking on 

 the posterior-dorsal slope. Adductor scars large, connected at their base 

 by a wide, entire pallial line. 



Authors have generally considered Phlyctiderma as a subgenus of 

 Diplodonta distinguished principally on the basis of its granulose or pustu- 

 late surface. However, in its peculiar hinge construction and deep immersion 

 of its ligament, it differs sufficiently for separate generic recognition. 

 Phlyctiderma has a stronger hinge, its bifid cardinal teeth larger, project- 

 ing or hook-shaped upward. The ligament is attached to a narrow, wedge- 

 shaped nymph which lies deeply immersed below the posterior-dorsal mar- 

 gin of the valve. Typical specimens have a granulose or nodose surface 

 sculpture, the nodes often V-shaped or elongated; some other species, such 

 as P. orhella (Gould) have no granules, the surface marked with concentric 

 riblets only. 



Phlyctiderma semiaspera (Philippi) Plate 32, figures 3-3c 



Diplodonta semiaspera Philippi, 1836, Wiegm. Arcli., vol. 1, p. 225, pi. 7, figs. 2a-d. 



