PANAMIC-PACIFIC PELECYPODA 193 



is short, its end rounded, the posterior side longer and higher, its margin 

 more flatly rounded so as to appear obscurely truncated and sometimes 

 lightly angulated. The beaks are small, prosogyrous, adjacent, touching 

 the hinge line, the umbones wide and full. The periostracum is a dull, dingy, 

 buffy-brown or olive-brown color, minutely, concentrically wrinkled or lamel- 

 lose scaly, retained generally on the lower part of the disk only, elsewhere 

 where worn-off, the exposed surface is corroded, chalky white, and in some 

 instances the corrosion has been so deep that the outer layer has been re- 

 moved and the shell substance reduced to a paper thinness. Hinge armature 

 is rather weak, the anterior lateral tooth placed close to the cardinals, the 

 posterior lateral tooth more distant, at the end of the hinge line above the 

 adductor scar. Surface of the disk below the periostracum is sculptured with 

 crowded, irregular, growth concentrics, the color white, more or less flushed 

 with pink except for a broad ray or band of tyrian purple along the pos- 

 terior-dorsal slope, this ray transmitted through the texture of the shell mto 

 the interior where its color is more intense; on the outside, this colored band 

 is seen only after the periostracum and the outer chalky layer have been 

 destroyed by corrosion. Interior of shell is white, except in the umbonal 

 cavity which may be faintly flushed with pink or apricot and the tyrian 

 purple ray previously mentioned along the posterior-dorsal area and across 

 the upper half of the posterior adductor scar. The pallial line is entire 

 except for a small sinus of variable size and shape near the posterior ad- 

 ductor scar. Size of shell varies within wide limits. 



Length 55 mm., height 40.5 mm., diameter 31.8 mm. Sua, Ecuador. 



Length 38,3 mm., height 29 mm., diameter 22.8 mm. Tumbez, Peru. 



This is a common and widely distributed species, and at Guayaquil, 

 is often served in hotels and restaurants as a seafood. The periostracum 

 is generally retained only around the ventral margin of the disk, the greater 

 part of the surface being corroded and revealing a chalky white shell under- 

 neath. In beach specimens, the periostracum and the outer layer is usually 

 removed, the shell then appearing soHd in texture. A distinctive character 

 of the species is the wide, purple band over the posterior-dorsal slope de- 

 veloped in the inner porcellaneous layer and hence seen only on deeply 

 corroded specimens; it always shows plainly in the interior. 



Pilsbry's P. zeteki was named from relatively small specimens from 

 Panama where it is locally common. Deshayes' name "notabiiis" was ap- 

 parently based on a beach worn specimen of fairly large size; this speci- 

 men was examined by Prime whose detailed description fits many of our 

 shells. 



Range — Costa Rica to northern Peru, Costa Rica: Puntarenas. Pan- 

 ama: Chame; Ecuador: Limones; Esmeraldas; Sua; Mompiche. Peru: 

 Tumbez. 



Folymesoda (Polymesoda) mexlcana (Broderlp and Sowerby) 



Plate 27, figures 8, 8a, 8b 



Cyrena mexicana Broderip and Sowerby, 1829, Zool. Journ,, vol, 4, p. 364. — Carpenter, 

 1857, Cat. Mazatlan Shells, Brit, Mus,, pp, 115, 116, No, 165 Mazatlan, Mexico,— 

 Prime, 1865, Smith, Misc, Coll., No, 145, p, 22, No, 18.— Sowerby, 1867, Conch. 

 Icon,, vol, 20, Cyrena, pi, 29, fig. 110 Mazatlan, Mexico, 



