PANAMIC-PACIFIC PELECYPODA 319 



flatly rounded at the end, the posterior side narrower. Texture of the shell 

 is extremely thin and fragile. Surface smooth, polished, undulated on the 

 anterior slope by fairly wide, concentric ribbons, less so on the posterior 

 slope. Hinge as typical for the genus. 



Length 10.4 mm., height 8.2 mm., diameter 2.4 mm., left valve, Holo- 

 type, ANSP 218964. 



This form is similar to C. subdiaphana (Carpenter) and perhaps but a 

 southern form of that species but its shell has a more rounded outline, 

 the two ends are almost alike. It is also similar to C. carpenteri Dall, 1900, 

 from the Miocene of Virginia, but differs somewhat in shape. 



The species is evidently rare, and known to me by a few free valves 

 only. 



Range — Panama. Panama: Bucaro. 



Family ? 



Genus HALODAKRA, new genus 



Type species, \ Circe subtrigona Carpenter. Recent, Panamic-Pacific 

 province. 



Shell small, ovate, veneriform, inequilateral, the posterior side longer 

 than the anterior, rounded at the ends. There is no evident lunule or es- 

 cutcheon. The ligament is internal or largely so, the resilifer represented 

 by a small, narrow scar along the lower or mner side of a short, nymphal 

 ridge; the tensilium, if present, is small and does not show above the valve 

 margin. Cardinal teeth have a radial arrangement under the beaks as in 

 the Veneridae, with two teeth in the right valve and three in the left; the 

 middle left cardinal tooth and the right anterior tooth are bifid. The 

 nymphal ridge in the right valve is a toothlike lamina. Ihe pallial line is 

 indistinct and is probably entire. Surface plain or sculptured only with 

 fine, concentric threads. Color mostly white or cream but with a large, 

 feather-like brown mark on the posterior slope, the color of which shows 

 through into the interior, together with small zigzag lines elsewhere. 



The familial affinities of this genus are unknown. Its internal liga- 

 ment removes it from the Veneridae which it resembles by its shape. Car- 

 penter, who described the type species, noted that the ligament was in- 

 ternal or deeply inset in the hinge plate; he tentatively referred the species 

 to the Astartidae. 



Halodakra subtrigrona (Carpenter) Plate 27, figures 1-lc 



}Circe subtrigona Carpenter, 1857, Cat. Mazatlan Shells, Brit. Mus., p. 82, No. 115 



Mazatlan, on Spondylus lamarckii. 

 Psephidia subtrigona (Carpenter), Myra Keen, 1958, Sea Shells of Tropical West 



America, p. 140, No. 317; also p. 622. 



A small species with the characters as described for the genus. Always 

 small, an average specimen measures: length 3.8 mm., height 3.2 mm., 

 diameter 1.8 mm. The species is easily recognized from other small bivalve 

 shells by the large, feather-like brown patch on its posterior slope, so dark 

 in color that its image is clearly seen on the inside of the valve. This small 

 species is often plentiful in fine, beach drift along with other small molluscan 

 shells. 



Range — Mexico to northern Peru. Mexico: Mazatlan. Panama: Pan- 

 ama City. Ecuador: Santa Elena, Punta Centinella. Peru: Mancora, Caleto 

 Sal. 



