192 AXEL A. OLSSON 



The Panamlc-Pacific species appear to belong to two principal genera. 



Key to genera of Corbiculidae 



I. Shell rounded or ovate, of thin or medium weight, generally inflated, 

 covered with a concentrically striated, wrinkled or fuzzy periostracum, 

 the surface beneath marked with growth hne concentrics only. Hinge 

 plate rather narrow. 



Genus Polyviesoda 



1. Rounded or ovate in shape, with a short, narrow pallial sinus not 

 crowded against the posterior adductor scar. 



Subgenus Polymesoda, s.s. 



2. Small, ovate-elongate shells, the pallial sinus ill-defined. 



Subgenus Pseudocyrena 

 (not regional) 



3. Shell larger, rounded, strongly convex. Pallial line entire or with a short, 

 ill-defined sinus pressed against the posterior adductor scar. 



Subgenus Egeta 



II. Shell rounded or trigonal, usually solid, with a wide hinge plate and 



coarse lateral teeth. Periostracum closely adherent, usually smooth or 



polished. Sculpture smooth or coarsely concentric. PalHal sinus short 



but usually well developed. 



Genus Neocyrena 



Genus POLYMESODA Rafinesque, 1820 

 (Leptosiphon Fischer, 1872, same type species as Polymesoda.) 



Type species by original designation, Cyclas caroliniana Bosc. Eastern 

 United States northward to Virginia. 



The shell is rounded trigonal, ovate or cordate, inflated, the beaks 

 prosogyrate and often corroded, the texture of the valves of medium or 

 heavy weight. Hinge with both cardinal and lateral teeth (the left an- 

 terior and middle teeth, and the right middle and posterior cardinal teeth 

 often bifid), the laterals equidistant. Surface smoothish or marked simply 

 with fine, concentric, growth line striae, and covered by a yellowish, greenish 

 or brownish periostracum, closely adherent or with narrowly spaced, elevated 

 or frilled lamellae. Surface color below the periostracum is white. Interior 

 with a short, narrow pallial sinus. 



Subgenus POLYMESODA, s.s. 

 With two, well-defined species in the Panamics-Pacific region. 



Polymesoda (Polymesoda) notabilis (Deshayes) Plate 27, figures 6, 6a: 



Plate 28, figure 6 



Cyrena notabilis Deshayes, 1854, Proc. Zool. Soc. London, p. 21 Peru (at the mouth of 

 rivers).— Prime, 1865, Smith. Misc. Coll., No. 145, p. 28, No. 29.— Sowerby, 

 1876, Conch. Icon., vol. 20, Cyrena, pi. 29, fig. 110 Payta, Peru. 



Polymesoda zeteki Pilsbry, 1931, Nautilus, vol. 44, No. 3, p. 85, pi. 7, figs. 2, 2a. Near 

 Chame, Panama. 



The shell is ovate, subrectangular, relatively thin, strongly inequi- 

 lateral, the beaks placed at the anterior one-third, moderately convex, the 

 posterior-umbonal slope rounded or obscurely angled. The anterior side 



