PANAMIC-PACIFIC PELECYPODA 335 



of the ligament or tensilium small and narrow, lodged along the dorsal mar- 

 gin just behind the beaks. Hinge teeth strong, on each side of the resiliferal 

 pit. Pallial sinus small but distinct and extends but a short distance beyond 

 the edge of the posterior adductor scar. Surface smooth or marked with 

 growth lines and covered by a skinlike periostracum, usually yellow and 

 glossy. Ventral margins smooth. 



Deshayes, who established the genus Mesodesma, cited Mactra donacia 

 as the first species although he did not actually designate it as the type. 

 Lamy divided the genus into several subgenera; most of its species live 

 in the Southern Hemisphere. 



Mesodesma donacinm (Lamarck) 



Mactra donacia Lamarck, 1818, Anim. s. Vert., vol. 5, p. 479. 



Donacilla chilensis d'Orbigny, 1846, Voy. Amer. Merid., Moll., p. 530. 



Mesodesma donacia (Lamarck), Hupe, 1854, in Gay, Hist. Chile. Zool., vol. 8, p. 352, 

 pi. 3, fig. 7. — Reeve, 1854, Conch. Icon., vol. 8, Mesodesma, pi. 2, fig. 11. — 

 Dail, 1909, Proc. U.S. Nat. Museum, vol. 37, No. 1704, pp. 161, 275, pi. 27, 

 fig. 1. — Lamy, 1914, Jour, de Conchyl., vol. 62, pp. 15, 16, text-fig. on page 5. 



Shell solid, broadly elongate, wedge-shape, the anterior side long, de- 

 pressed towards the end which is rounded, the posterior side short, its 

 margin sloping downward sharply and forming with the dorsal margin at 

 the beak a right angle bend. Surface smooth or with concentric markings, 

 its color white, covered with a straw-yellow periostracum. 



This species is not a member of the Panamic-Pacific fauna. It is com- 

 mon along the coast of Peru from Lima southward to Chile. Known as 

 "Almejas", it is much used for both food and bait, often sold in the local 

 markets. 



Range — Peruvian cold-water province from near Trujillo southward 

 to central Chile. Dall's record of this species from Sechura Bay is question- 

 able. 



Genus ERYILIA Turton, 1822 

 Type species by monotypy, Mya nitens Montagu. Recent, West Indies 

 and Florida. 



Shell small, ovate to subtrigonal, with the beaks and umbones placed 

 near the middle line, but with the posterior side usually a little longer, its 

 dorsal margin straight, sloping down towards a pointed or bluntly rounded 

 end. The beaks are small, inconspicuous, and slightly opisthogyrate. E.x- 

 ternal ligament small or obsolete, the internal ligament strong, attached 

 to a deep, Mactra-\'\ke resilifer under the beak. The right valve has a strong, 

 posterior cardinal tooth bordered by a narrow socket; the left valve has a 

 posterior lamellar tooth along the hinge border with a deep cardinal socket 

 below it; there are no laterals. Pallial sinus large, ample, rounded at the 

 end, and reaching into the middle of the shell cavity, the inner margins 

 of the valves smooth. Surface smooth or marked with strong, concentric 

 threads. 



Several species of Ervilia occur in the West Atlantic and Caribbean 

 region and they are usually plentiful at all localities. As yet unknown in 



