334 AXEL A. OLSSON 



Standella thracoides (Adams and Reeve), Lamy, 1917, Jour, de Conch., vol. 63, p. 386 



reports a valve from Machala, Ecuador. 

 Tumbeziconcha thracoides (Adams and Reeve), Pilsbry and Olsson, 1935, Nautilus, 



vol. 48, No. 4, pp. 119-121, pi. 6, fig. 9. 



In outline, the shell is subelliptical, short, with the beaks placed in 

 front of the middle so that the posterior side is somewhat longer, narrower 

 than the anterior. The external sculpture is produced by deep waves or 

 undulations which unlike those of Harvella and Ra'eta are not strictly con- 

 centric to the margin over the whole surface but are oblique on the middle 

 and posterior sides. The posterior-dorsal area is set apart by the sudden 

 ending of the concentric undulations and not by a ridge or keel. In addition 

 to the surface undulations, the whole disk is thickly covered with incre- 

 mental, concentric wrinkles giving the effect as if it had been painted with 

 a coarse-haired brush. There is sometimes a faint line or small ridge ex- 

 tending along the anterior side of the umbone to the ventral margin. The 

 shell is usually small, ranging in length from 30 to 43 mm. Some average 

 measurements as follows: 



Length 42.5 mm., height 31.7 mm., semidiameter 10 mm. right valve. 



Length 34.0 mm., height 26.2 mm., semidiameter 8 mm. left valve. 



This species was described erroneously as from the "Eastern Pacific" 

 without indication of exact locality, but in all probability the original speci- 

 mens were obtained from some station in the Panamic-Pacific region. It is 

 most commonly found on or near mud flats fronting on mangrove swamps 

 and in such situations, the species may be plentiful. 



Range — Gulf of Fonseca, El Salvador to northern Peru. El Salvador; 

 La Union (Lowe). Panama: San Miguel, Rey Island, Pearl Islands. Colom- 

 bia: Isla del Gallo. Peru: Tumbez. 



Family MESODESMATIDAE 



Shell medium or large-sized, solid, donaciform, strongly inequilateral, 

 the anterior side much longer than the posterior, its external surface smooth, 

 usually polished and covered in life with a thick, often glossy periostracum; 

 also including a group of small shells of trigonal form and nearly median 

 beaks. The hinge is much like that of the Mactridae, the ligament largely 

 internal, lodged in a large deep pit or chondrophore bordered by the cardinal 

 and lateral teeth. The pallial sinus is small. 



This family is mainly distinguished from the Mactridae on anatomical 

 grounds and by its free, naked, retractible siphons. 



I. Shell large, donaciform, the anterior side much longer than the pos- 

 terior. Cold waters of the Peruvian province. 



Genus Mesodesma 



II. Shell small, subtrigonal, and subequilateral. 



Genus Ervilia 

 Genus MESODES^IA Deshayes, 1830 

 {Ceronia Gray, 1853.) 

 Type species by subsequent designation, Anton, 1839, Mactra donacia 

 Lamarck. 



Shell solid, elongate subquadrate or donaciform, the posterior side 

 shorter, its end as if obliquely truncated. Hinge strong, the ligament mostly 

 internal, attached to a large, deep pit or chondrophore, the external section 



