PANAMIC-PACIFIC PELECYPODA 433 



of the visceral cavity. Adductor scars are of nearly equal size, placed on 

 the thickened marginal zone; the pallial sinus is small. 



Length 21.4 mm., height 12.3 mm., diameter 9.6 mm. Esmeraldas, 

 Ecuador. Holotype, ANSP 218903. 



A depressed ray extends from the beak across the middle of the disk 

 to the ventral margin. This Recent species is related to the Miocene C. 

 hexacyma Brown and Pilsbry from the Gatun Miocene as well as to C. 

 cruziana Olsson from the Miocene of Peru, but is proportionately longer 

 and the depressed ray is deeper. 



Range — Coast of Ecuador. Ecuador: off Esmeraldas in about 20 feet 

 of water. 



Genus SERRACOEBULA, new genus 



Type species, Serracorbula tumaca, new species. Southwestern Colombia. 



Shell solid and heavy, subequivalve, convex, the anterior end well 

 rounded, the posterior with a rostrum in each valve, its end produced into 

 a short, narrow beak. The surface sculpture is similar in each valve. Ven- 

 tral margin is evenly serrated or striated all around. Visceral cavity, deep 

 and round, its depth emphasized by thickening of the marginal zone below 

 the pallial line. The adductor scars are large and distinct. 



Serracorbula tumaca, new species Plate 76, figures 4-4d 



Shell of medium size (length about 12 mm.), ovate, strongly convex, 

 with high, central umbones and beaks, the valves subequilateral except 

 for the short, twisted, posterior snout or beak. Surface sculpture consists 

 of fine or medium, close-set, concentric riblets. In the adult, the shell 

 becomes much thickened, its ventral side turned sharply inward to form a 

 flattened area, the growing edge formed by a dark, brown layer; in the 

 right valve, this layer is striated transversely on the inner side, and on the 

 outer side in the left valve; along the margin itself, these striations form 

 fine, even crenulations except at the hinge. 



Length 12.3 mm., height 7.3 mm., semidiameter 4.8 mm. A right valve. 

 Tumaco, Colombia. Holotype, ANSP 218948. 



A species of tidal mud flats, and easily recognized by its serrated 

 margins, usually brown. 



Rarige — Panama southward to Peru. Panama: Rey Island, Pearl 

 Islands. Colombia: Tumaco. Ecuador: Limones. Peru: Tumbez. 



Genus TENUICORBULA Olsson, 1932 

 Type species by original designation, Corbula tenuis Sowerby. Pacific 

 coast of Central America. 



Shell elongated, subrectangular, inequilateral, the anterior side usually 

 a little longer, higher and wider, the posterior side contracted and bearing 

 a strong keel which encloses a large rostral area, the texture of the shell 

 usually thin. Valves are subequal in size, shape, and sculpture. No lunule, 

 but there is a large, sunken escutcheon in each valve. Hinge as in Caryocor- 

 bula, the cardinal tooth of the right valve large, curved upward, or hook- 



