PANAMIC-PACIFIC PELECYPODA 375 



This recently described species is widely distributed and fairly common 

 in its proper environment, mostly on mud flats near mangrove swamps. 

 Fresh shells have a slightly frosted surface due to minute granules, most 

 heavily developed over the posterior slope but beach specimens are generally 

 smooth due to wear. 



Range — Panama to northwestern Peru. Panama: Garachine on San 

 Miguel Bay. Colombia: Isla del Gallo. Ecuador: Esmeraldas. Peru: Puerto 

 Pizarro at the mouth of the Tumbez River. 



Genus SEMELINA Dall, 1900 



Type species by original designation, Semelina nuculoides (Conrad). 

 Miocene and Recent. Southeastern coast of the United States. 



The shell is small, nuculiform, elliptical, with the posterior side much 

 shorter than the anterior. The hinge is characterized by the strong, lateral 

 sockets in the right valve, the left valve without distinct, lateral teeth, the 

 dorsal margins fitting into the opposite sockets; each valve has a central, 

 bifid cardinal tooth, the resilifer or attachment scar of the internal ligament 

 behind them. Surface sculptured with close, concentric threads. 



This is a genus of small shells belonging to several species, common 

 and widely distributed in the western Atlantic. A single species known from 

 the Pacific zone. 



Semelina subqoadrata (Carpenter) Plate 66, figure 11 



IMontacuta subquadrata Carpenter, 1857, Cat. Mazatlan Shells, Brit. Mus., p. 113, sp. 

 162. 



Shell small, seldom above 5 mm., ovate-elliptical, relatively solid, and 

 of low but uniform inflation. Umbones low, ending in small, acutely pointed, 

 opisthogyrate beaks accentuated by a pronounced flattening or excavation 

 of the posterior dorsal margin below them. Hinge as normal for the genus, 

 the lateral sockets of the right valve being strong. Pallial sinus large, 

 rounded, almost reaching to the anterior adductor, below the lower limb 

 of the sinus is about half confluent with the pallial line which as a simple 

 line extends well beyond its junction. Color generally white, more rarely 

 flushed with apricot-pink. Sculpture over most of the anterior and middle 

 surface of the valves is formed by even, close-set, threadlike concentrics 

 but on the posterior umbonal slope, the alternate threads end sharply, the 

 others increasing in strength, the resulting sculpture, thereby, becoming 

 finely lamellose. 



Length 5.3 mm., height 3.8 mm., diameter 1 mm. (a left valve). 



This species is closely similar to the West Atlantic, S. nuculoides Conrad. 

 Range — Mexico to Colombia. Mexico: Mazatlan (Carpenter). Panama: 

 El Lagartillo; Puerto Mensabi; Pearl Islands. Colombia: Isla del Gallo. 



Family TELLINEDAE 



The shell varies much in shape and size with narrowly elongated to 



broadly orbicular valves, generally inequilateral, the anterior side being 



the longer. Valves are usually unequal, the left valve typically the larger, 



more strongly convex, the posterior side often narrow and produced, and 



