PANAMIC-PACIFIC PELECYPODA 117 



ribs, usually corroded, producing rather coarse crenulation around the 

 posterior margin, finer ones along the ventral margin and often becoming 

 obsolete there. Surface color usually purplish over the posterior surface and 

 white on the anterior-ventral side. Dorsal margin above the hgament not 

 crenulated. The anterior margin below the beak is turned backwards along 

 the hinge and terminates in a sharp stump as if cut off abruptly; it marks 

 the forward end of the ligament. The rounded end of the anterior margin in 

 front of the beak has several small teeth or crenulations. Posterior adductor 

 scar rounded, joined with the narrow, linear retractor scar, together forming 

 a pipe-shaped impression. 



The type species has been referred to Brachidontes but it differs by 

 important hinge characters as well as in other features. 



PerumytilDS purpuratus (Lamarck) Plate 12, figure 1; Plate 14, figures 1-lb 



Modiola purpurata Lamarck, 1819, Anim. s. Vert., vol. 6, p. 113. 



Mytilus ovalis Lamarck, 1819, idem., p. 121. — Encylop. Meth., pi. 219, figs. 3a, 3b. — 



d'Orbigny, 1846, Voy. Amer. Merid., vol. 5, p. 648. 

 Modiolus purpuratus (Lamarck), Dall, 1910, Proc. U. S. Nat. Museum, vol. 37, No. 



1704, pp. 153, 258. 

 Brachidontes Purpuratus (Lamarck), Soot-Ryen, 1955, Allan Hancock Expeditions, vol. 



20, No. 1, p. 45, pi. 4, fig. 18, text-fig. 30. 



Shell small or of medium size (length up to about 40 mm.), elongately 

 ovate, convex, narrowed and sometimes deeply compressed along the an- 

 terior-ventral side. The sculpture is formed by coarsely wrinkled radial 

 riblets, rather large on the rounded posterior slope, finer on the ventral side. 

 Color is a dark purple to nearly black over the posterior surface, often 

 white on the anterior part. The umbonal slope is often worn nearly smooth 

 by exposure to the heavy surf waves. 



This is a common species of the cooler Peruvian coastal fauna from 

 Callao southward; northward it seems to be much rarer as well as smaller 

 in size. A few specimens were collected on the island of Lobos de Tierra and 

 at Yasila near Paita. Dall has recorded the species from Estero Zarumilla 

 on the Ecuadorian border near Capon but we have never seen specimens 

 from so far north. 



Range — Gulf of Guayaquil southward to Chile. Ecuador: Estero 

 Zarumilla (Dall). Peru: Yasila near Paita; Lobos de Tierra; Callao and 

 southward (common). 



Genus BRACHIDONTES Swainson, 1840 



Type species by monotypy, Modiola sulcata Lamarck, 1819 (Encyclop. 

 Meth., pi. 220, figs. 2a, 2b). 



Shell small or of medium size, mytiliform, with the small beaks not 

 quite terminal and from which there extends posteriorly an umbonal in- 

 flation and along which lies the long axis of the shell. Posterior-dorsal or 

 hinge margin straight or weakly curved, usually ending distally in a small 

 angle, and more or less strongly crenulated along its whole length. Hinge 

 margin on the short anterior side has two or more, small, dysodont teeth. 

 Ligament subinternal, its resilifer forming a narrow scar about half the 

 length of the posterior-dorsal margin. The adductor scars are distinct, the 



